diff --git a/app/main.py b/app/main.py
index ead5400..1c7329c 100644
--- a/app/main.py
+++ b/app/main.py
@@ -47,6 +47,7 @@ from app.routes import (
     sequences,
     system_settings,
     attachments,
+    custom_fields,
 )
 
 
@@ -304,6 +305,7 @@ def create_app() -> FastAPI:
     app.include_router(attachments.router)
     app.include_router(addresses.router)
     app.include_router(audit.router)
+    app.include_router(custom_fields.router)
 
     # ── Register plugin routes (before SPA catch-all) ──────────────────
     registry = get_registry()
diff --git a/app/plugins/manifest.py b/app/plugins/manifest.py
index 845dce2..421c549 100644
--- a/app/plugins/manifest.py
+++ b/app/plugins/manifest.py
@@ -138,6 +138,21 @@ class FrontendDashboardWidget(BaseModel):
     permission: str = Field(default="", description="Optional permission required")
 
 
+class CustomFieldDefinition(BaseModel):
+    """A custom field definition contributed by a plugin manifest."""
+
+    name: str = Field(..., min_length=1, max_length=80, description="Unique field name (snake_case)")
+    label: str = Field(default="", description="Human-readable label (fallback if i18n key missing)")
+    label_key: str = Field(default="", description="i18n key for the field label")
+    field_type: str = Field(
+        ..., pattern="^(text|number|date|select|multiselect|boolean)$", description="Field type"
+    )
+    options: list[str] = Field(default_factory=list, description="Options for select/multiselect types")
+    default_value: Any = Field(default=None, description="Default value for the field")
+    required: bool = Field(default=False, description="Whether the field is required")
+    entity: str = Field(default="contact", description="Entity type this field applies to (contact/file/etc)")
+
+
 class MiniAppContribution(BaseModel):
     """A MiniApp contributed by a plugin manifest."""
 
@@ -214,6 +229,9 @@ class PluginManifest(BaseModel):
     miniapps: list[MiniAppContribution] = Field(
         default_factory=list, description="MiniApps contributed by this plugin"
     )
+    custom_fields: list[CustomFieldDefinition] = Field(
+        default_factory=list, description="Custom field definitions contributed by this plugin"
+    )
 
 
     @field_validator("name")
diff --git a/app/plugins/registry.py b/app/plugins/registry.py
index 3182139..c8b4851 100644
--- a/app/plugins/registry.py
+++ b/app/plugins/registry.py
@@ -760,6 +760,7 @@ class PluginRegistry:
                     "detail_tabs": [tab.model_dump() for tab in m.detail_tabs],
                     "settings_pages": [page.model_dump() for page in m.settings_pages],
                     "dashboard_widgets": [widget.model_dump() for widget in m.dashboard_widgets],
+                    "custom_fields": [cf.model_dump() for cf in m.custom_fields],
                 }
             )
         return manifests
diff --git a/app/routes/custom_fields.py b/app/routes/custom_fields.py
new file mode 100644
index 0000000..92a3374
--- /dev/null
+++ b/app/routes/custom_fields.py
@@ -0,0 +1,158 @@
+"""Custom fields routes — merge plugin definitions with stored values."""
+
+from __future__ import annotations
+
+import uuid
+from typing import Any
+
+from fastapi import APIRouter, Depends, HTTPException, Body
+from pydantic import BaseModel
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.db import get_db
+from app.deps import get_current_user, require_permission
+from app.models.contact import Contact
+from app.plugins.registry import get_registry
+
+router = APIRouter(prefix="/api/v1/contacts", tags=["custom-fields"])
+
+
+class CustomFieldUpdateRequest(BaseModel):
+    """Request body for updating custom field values."""
+
+    values: dict[str, Any] = {}
+
+
+def _collect_custom_field_definitions(entity: str = "contact") -> list[dict[str, Any]]:
+    """Collect all custom field definitions from active plugin manifests."""
+    definitions: list[dict[str, Any]] = []
+    seen_names: set[str] = set()
+    registry = get_registry()
+    for plugin in registry._plugins.values():
+        manifest = plugin.manifest
+        for cf in manifest.custom_fields:
+            if cf.entity != entity:
+                continue
+            if cf.name in seen_names:
+                continue
+            seen_names.add(cf.name)
+            definitions.append(
+                {
+                    "name": cf.name,
+                    "label": cf.label,
+                    "label_key": cf.label_key,
+                    "field_type": cf.field_type,
+                    "options": cf.options,
+                    "default_value": cf.default_value,
+                    "required": cf.required,
+                    "entity": cf.entity,
+                    "plugin": manifest.name,
+                }
+            )
+    return definitions
+
+
+def _merge_definitions_with_values(
+    definitions: list[dict[str, Any]], stored: dict[str, Any] | None
+) -> list[dict[str, Any]]:
+    """Merge field definitions with stored values, applying defaults."""
+    stored = stored or {}
+    result: list[dict[str, Any]] = []
+    for d in definitions:
+        name = d["name"]
+        value = stored.get(name, d.get("default_value"))
+        entry = {**d, "value": value}
+        result.append(entry)
+    return result
+
+
+@router.get("/{contact_id}/custom-fields", dependencies=[Depends(require_permission("contacts:read"))])
+async def get_custom_fields(
+    contact_id: str,
+    db: AsyncSession = Depends(get_db),
+    current_user: dict = Depends(get_current_user),
+):
+    """Get all custom fields for a contact (merged definitions + stored values)."""
+    tenant_id = uuid.UUID(current_user["tenant_id"])
+    try:
+        cid = uuid.UUID(contact_id)
+    except (ValueError, TypeError):
+        raise HTTPException(400, detail={"detail": "Invalid contact_id", "code": "invalid_id"}) from None
+
+    result = await db.execute(
+        select(Contact).where(Contact.id == cid, Contact.tenant_id == tenant_id)
+    )
+    contact = result.scalar_one_or_none()
+    if contact is None:
+        raise HTTPException(404, detail={"detail": "Contact not found", "code": "not_found"})
+
+    definitions = _collect_custom_field_definitions("contact")
+    merged = _merge_definitions_with_values(definitions, contact.custom)
+    return {"fields": merged}
+
+
+@router.patch("/{contact_id}/custom-fields", dependencies=[Depends(require_permission("contacts:write"))])
+async def update_custom_fields(
+    contact_id: str,
+    body: CustomFieldUpdateRequest,
+    db: AsyncSession = Depends(get_db),
+    current_user: dict = Depends(get_current_user),
+):
+    """Update custom field values for a contact (stored in contacts.custom JSONB)."""
+    tenant_id = uuid.UUID(current_user["tenant_id"])
+    try:
+        cid = uuid.UUID(contact_id)
+    except (ValueError, TypeError):
+        raise HTTPException(400, detail={"detail": "Invalid contact_id", "code": "invalid_id"}) from None
+
+    result = await db.execute(
+        select(Contact).where(Contact.id == cid, Contact.tenant_id == tenant_id)
+    )
+    contact = result.scalar_one_or_none()
+    if contact is None:
+        raise HTTPException(404, detail={"detail": "Contact not found", "code": "not_found"})
+
+    # Validate against definitions
+    definitions = _collect_custom_field_definitions("contact")
+    def_map = {d["name"]: d for d in definitions}
+
+    current_custom = dict(contact.custom or {})
+    for name, value in body.values.items():
+        if name not in def_map:
+            raise HTTPException(
+                400,
+                detail={"detail": f"Unknown custom field: {name}", "code": "unknown_field"},
+            )
+        field_def = def_map[name]
+        # Validate required
+        if field_def["required"] and (value is None or value == ""):
+            raise HTTPException(
+                400,
+                detail={"detail": f"Field '{name}' is required", "code": "required_field"},
+            )
+        # Validate select/multiselect options
+        if field_def["field_type"] == "select" and value is not None:
+            if value not in field_def["options"]:
+                raise HTTPException(
+                    400,
+                    detail={"detail": f"Invalid option for field '{name}'", "code": "invalid_option"},
+                )
+        if field_def["field_type"] == "multiselect" and value is not None:
+            if not isinstance(value, list):
+                raise HTTPException(
+                    400,
+                    detail={"detail": f"Field '{name}' must be a list", "code": "invalid_type"},
+                )
+            for v in value:
+                if v not in field_def["options"]:
+                    raise HTTPException(
+                        400,
+                        detail={"detail": f"Invalid option '{v}' for field '{name}'", "code": "invalid_option"},
+                    )
+        current_custom[name] = value
+
+    contact.custom = current_custom
+    await db.flush()
+    merged = _merge_definitions_with_values(definitions, contact.custom)
+    return {"fields": merged}
diff --git a/dump.rdb b/dump.rdb
new file mode 100644
index 0000000..9fb73a8
Binary files /dev/null and b/dump.rdb differ
diff --git a/frontend/src/__tests__/CustomFieldRenderer.test.tsx b/frontend/src/__tests__/CustomFieldRenderer.test.tsx
new file mode 100644
index 0000000..21b07c8
--- /dev/null
+++ b/frontend/src/__tests__/CustomFieldRenderer.test.tsx
@@ -0,0 +1,147 @@
+/**
+ * CustomFieldRenderer tests — renders custom fields based on field_type.
+ */
+
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/react';
+import { CustomFieldRenderer } from '@/components/contacts/CustomFieldRenderer';
+import type { CustomFieldDefinition } from '@/api/customFields';
+
+// Mock i18n
+vi.mock('react-i18next', () => ({
+  useTranslation: () => ({ t: (key: string) => key }),
+}));
+
+// Mock UI components
+vi.mock('@/components/ui/Input', () => ({
+  Input: ({ value, onChange, type, id, ...props }: any) => (
+    <input
+      data-testid={props['data-testid'] || `input-${id}`}
+      type={type || 'text'}
+      value={value || ''}
+      onChange={onChange}
+    />
+  ),
+}));
+
+vi.mock('@/components/ui/Select', () => ({
+  Select: ({ value, onChange, id, options, children, ...props }: any) => (
+    <select
+      data-testid={props['data-testid'] || `select-${id}`}
+      value={value || ''}
+      onChange={onChange}
+    >
+      {options ? options.map((o: any) => <option key={o.value} value={o.value}>{o.label}</option>) : children}
+    </select>
+  ),
+}));
+
+const mockFields: CustomFieldDefinition[] = [
+  {
+    name: 'lead_source',
+    label: 'Lead Source',
+    label_key: 'custom.leadSource',
+    field_type: 'select',
+    options: ['website', 'referral', 'cold_call'],
+    default_value: null,
+    required: false,
+    entity: 'contact',
+    plugin: 'test_plugin',
+    value: 'website',
+  },
+  {
+    name: 'score',
+    label: 'Score',
+    label_key: 'custom.score',
+    field_type: 'number',
+    options: [],
+    default_value: 0,
+    required: false,
+    entity: 'contact',
+    plugin: 'test_plugin',
+    value: 42,
+  },
+  {
+    name: 'active',
+    label: 'Active',
+    label_key: 'custom.active',
+    field_type: 'boolean',
+    options: [],
+    default_value: false,
+    required: false,
+    entity: 'contact',
+    plugin: 'test_plugin',
+    value: true,
+  },
+];
+
+describe('CustomFieldRenderer', () => {
+  it('renders read mode with field values', () => {
+    render(<CustomFieldRenderer fields={mockFields} mode="read" />);
+    expect(screen.getByTestId('custom-fields-read')).toBeInTheDocument();
+    expect(screen.getByText('website')).toBeInTheDocument();
+    expect(screen.getByText('42')).toBeInTheDocument();
+    expect(screen.getByText('✓')).toBeInTheDocument();
+  });
+
+  it('renders edit mode with form controls', () => {
+    render(
+      <CustomFieldRenderer
+        fields={mockFields}
+        mode="edit"
+        values={{ lead_source: 'website', score: 42, active: true }}
+        onChange={vi.fn()}
+      />
+    );
+    expect(screen.getByTestId('custom-fields-edit')).toBeInTheDocument();
+    expect(screen.getByTestId('select-cf-lead_source')).toBeInTheDocument();
+    expect(screen.getByTestId('input-cf-score')).toBeInTheDocument();
+    expect(document.getElementById('cf-active')).toBeInTheDocument();
+  });
+
+  it('calls onChange when select value changes', () => {
+    const onChange = vi.fn();
+    render(
+      <CustomFieldRenderer
+        fields={[mockFields[0]]}
+        mode="edit"
+        values={{ lead_source: 'website' }}
+        onChange={onChange}
+      />
+    );
+    const select = screen.getByTestId('select-cf-lead_source');
+    fireEvent.change(select, { target: { value: 'referral' } });
+    expect(onChange).toHaveBeenCalledWith('lead_source', 'referral');
+  });
+
+  it('renders nothing when fields array is empty', () => {
+    const { container } = render(<CustomFieldRenderer fields={[]} mode="read" />);
+    expect(container.firstChild).toBeNull();
+  });
+
+  it('renders multiselect field with checkboxes', () => {
+    const multiField: CustomFieldDefinition = {
+      name: 'tags',
+      label: 'Tags',
+      label_key: 'custom.tags',
+      field_type: 'multiselect',
+      options: ['vip', 'customer', 'lead'],
+      default_value: [],
+      required: false,
+      entity: 'contact',
+      plugin: 'test_plugin',
+      value: ['vip'],
+    };
+    render(
+      <CustomFieldRenderer
+        fields={[multiField]}
+        mode="edit"
+        values={{ tags: ['vip'] }}
+        onChange={vi.fn()}
+      />
+    );
+    const checkboxes = screen.getAllByRole('checkbox');
+    expect(checkboxes).toHaveLength(3);
+    expect(checkboxes[0]).toBeChecked();
+  });
+});
diff --git a/frontend/src/api/customFields.ts b/frontend/src/api/customFields.ts
new file mode 100644
index 0000000..8728b2d
--- /dev/null
+++ b/frontend/src/api/customFields.ts
@@ -0,0 +1,42 @@
+/**
+ * Custom fields API hooks — merge plugin definitions with stored values.
+ */
+
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
+import { apiGet, apiPatch } from './client';
+
+export interface CustomFieldDefinition {
+  name: string;
+  label: string;
+  label_key: string;
+  field_type: 'text' | 'number' | 'date' | 'select' | 'multiselect' | 'boolean';
+  options: string[];
+  default_value: any;
+  required: boolean;
+  entity: string;
+  plugin: string;
+  value: any;
+}
+
+export interface CustomFieldsResponse {
+  fields: CustomFieldDefinition[];
+}
+
+export function useCustomFields(contactId?: string) {
+  return useQuery({
+    queryKey: ['custom-fields', contactId],
+    queryFn: () => apiGet<CustomFieldsResponse>(`/contacts/${contactId}/custom-fields`),
+    enabled: !!contactId,
+  });
+}
+
+export function useUpdateCustomFields() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: ({ contactId, values }: { contactId: string; values: Record<string, any> }) =>
+      apiPatch<CustomFieldsResponse>(`/contacts/${contactId}/custom-fields`, { values }),
+    onSuccess: (_data, variables) => {
+      queryClient.invalidateQueries({ queryKey: ['custom-fields', variables.contactId] });
+    },
+  });
+}
diff --git a/frontend/src/components/contacts/ContactDetail.tsx b/frontend/src/components/contacts/ContactDetail.tsx
index 7142b99..9fbdc75 100644
--- a/frontend/src/components/contacts/ContactDetail.tsx
+++ b/frontend/src/components/contacts/ContactDetail.tsx
@@ -14,6 +14,8 @@ import { usePluginStore } from '@/store/pluginStore';
 import { useAIUIControlStore } from '@/store/aiUIControlStore';
 import { PluginPage } from '@/components/plugins/PluginLoader';
 import { useAuthStore } from '@/store/authStore';
+import { CustomFieldRenderer } from '@/components/contacts/CustomFieldRenderer';
+import { useCustomFields } from '@/api/customFields';
 import {
   type UnifiedContact,
   type ContactPerson,
@@ -177,6 +179,10 @@ export function ContactDetail({ contact, loading, onEdit, onDeleted }: ContactDe
   }, [aiActiveModal]);
   const user = useAuthStore(s => s.user);
 
+  // Custom fields from plugin definitions
+  const customFieldDefs = usePluginStore(s => s.getCustomFieldsForEntity('contact'));
+  const { data: customFieldsData } = useCustomFields(contact?.id);
+
   if (loading) {
     return (
       <div className="flex items-center justify-center py-12" data-testid="contact-detail-loading">
@@ -420,11 +426,12 @@ export function ContactDetail({ contact, loading, onEdit, onDeleted }: ContactDe
         </Section>
 
         {/* Custom Fields */}
-        {contact.custom && Object.keys(contact.custom).length > 0 && (
+        {contact.id && customFieldDefs.length > 0 && (
           <Section title={t('contacts.customFields')}>
-            <pre className="text-xs text-secondary-700 bg-secondary-50 rounded p-2 overflow-x-auto">
-              {JSON.stringify(contact.custom, null, 2)}
-            </pre>
+            <CustomFieldRenderer
+              fields={customFieldsData?.fields || []}
+              mode="read"
+            />
           </Section>
         )}
 
diff --git a/frontend/src/components/contacts/ContactEditModal.tsx b/frontend/src/components/contacts/ContactEditModal.tsx
index f9e7891..ea3a588 100644
--- a/frontend/src/components/contacts/ContactEditModal.tsx
+++ b/frontend/src/components/contacts/ContactEditModal.tsx
@@ -13,6 +13,9 @@ import {
   useCreateUnifiedContact,
   useUpdateUnifiedContact,
 } from '@/api/hooks';
+import { CustomFieldRenderer } from '@/components/contacts/CustomFieldRenderer';
+import { useCustomFields, useUpdateCustomFields } from '@/api/customFields';
+import { usePluginStore } from '@/store/pluginStore';
 
 export interface ContactEditModalProps {
   open: boolean;
@@ -105,6 +108,26 @@ export function ContactEditModal({ open, onClose, contact, onSaved }: ContactEdi
 
   const currentType = watch('type');
 
+  // Custom fields
+  const customFieldDefs = usePluginStore(s => s.getCustomFieldsForEntity('contact'));
+  const { data: customFieldsData } = useCustomFields(contact?.id);
+  const updateCustomFields = useUpdateCustomFields();
+  const [customValues, setCustomValues] = React.useState<Record<string, any>>({});
+
+  React.useEffect(() => {
+    if (open && customFieldsData?.fields) {
+      const vals: Record<string, any> = {};
+      for (const f of customFieldsData.fields) {
+        vals[f.name] = f.value ?? f.default_value ?? null;
+      }
+      setCustomValues(vals);
+    }
+  }, [open, customFieldsData]);
+
+  const handleCustomFieldChange = (name: string, value: any) => {
+    setCustomValues(prev => ({ ...prev, [name]: value }));
+  };
+
   // Reset form when modal opens
   useEffect(() => {
     if (open) {
@@ -164,15 +187,27 @@ export function ContactEditModal({ open, onClose, contact, onSaved }: ContactEdi
     };
 
     try {
+      let savedId: string | undefined;
       if (isEdit && contact) {
         await updateMutation.mutateAsync({ id: contact.id, data });
+        savedId = contact.id;
         toast.success(t('contacts.updated'));
         onSaved?.(contact.id);
       } else {
         const result = await createMutation.mutateAsync(data) as { id: string };
+        savedId = result.id;
         toast.success(t('contacts.created'));
         onSaved?.(result.id);
       }
+      // Save custom fields if any definitions exist and we have a contact ID
+      if (savedId && customFieldDefs.length > 0 && Object.keys(customValues).length > 0) {
+        try {
+          await updateCustomFields.mutateAsync({ contactId: savedId, values: customValues });
+        } catch (cfErr: any) {
+          // Don't fail the whole save if custom fields fail
+          console.error('Custom fields save failed:', cfErr);
+        }
+      }
       onClose();
     } catch (err: any) {
       toast.error(err.message || (isEdit ? t('contacts.updateFailed') : t('contacts.createFailed')));
@@ -281,6 +316,19 @@ export function ContactEditModal({ open, onClose, contact, onSaved }: ContactEdi
           </div>
         </div>
 
+        {/* Custom Fields */}
+        {customFieldDefs.length > 0 && (
+          <div className="border border-secondary-200 rounded-lg p-3">
+            <h3 className="text-sm font-semibold text-secondary-700 mb-2">{t('contacts.customFields')}</h3>
+            <CustomFieldRenderer
+              fields={customFieldsData?.fields || customFieldDefs.map(d => ({ ...d, value: d.default_value, plugin: '' }))}
+              mode="edit"
+              values={customValues}
+              onChange={handleCustomFieldChange}
+            />
+          </div>
+        )}
+
         {/* Actions */}
         <div className="flex justify-end gap-2 pt-2">
           <Button variant="secondary" onClick={onClose}>{t('common.cancel')}</Button>
diff --git a/frontend/src/components/contacts/CustomFieldRenderer.tsx b/frontend/src/components/contacts/CustomFieldRenderer.tsx
new file mode 100644
index 0000000..b84e9a5
--- /dev/null
+++ b/frontend/src/components/contacts/CustomFieldRenderer.tsx
@@ -0,0 +1,146 @@
+/**
+ * CustomFieldRenderer — renders custom fields based on field_type.
+ * Supports read-only and editable modes.
+ */
+
+import React from 'react';
+import { useTranslation } from 'react-i18next';
+import { Input } from '@/components/ui/Input';
+import { Select } from '@/components/ui/Select';
+import type { CustomFieldDefinition } from '@/api/customFields';
+
+export interface CustomFieldRendererProps {
+  fields: CustomFieldDefinition[];
+  mode: 'read' | 'edit';
+  values?: Record<string, any>;
+  onChange?: (name: string, value: any) => void;
+}
+
+function formatValue(value: any, fieldType: string): string {
+  if (value === null || value === undefined || value === '') return '—';
+  if (fieldType === 'boolean') return value ? '✓' : '✗';
+  if (fieldType === 'multiselect' && Array.isArray(value)) return value.join(', ');
+  if (fieldType === 'date' && value) {
+    try {
+      return new Date(value).toLocaleDateString();
+    } catch {
+      return String(value);
+    }
+  }
+  return String(value);
+}
+
+export function CustomFieldRenderer({ fields, mode, values = {}, onChange }: CustomFieldRendererProps) {
+  const { t } = useTranslation();
+
+  if (!fields || fields.length === 0) return null;
+
+  if (mode === 'read') {
+    return (
+      <dl className="grid grid-cols-2 gap-3" data-testid="custom-fields-read">
+        {fields.map((field) => {
+          const label = field.label_key ? t(field.label_key) : field.label || field.name;
+          const value = values[field.name] ?? field.value;
+          return (
+            <div key={field.name}>
+              <dt className="text-xs font-medium text-secondary-500">{label}</dt>
+              <dd className="text-sm text-secondary-900">{formatValue(value, field.field_type)}</dd>
+            </div>
+          );
+        })}
+      </dl>
+    );
+  }
+
+  return (
+    <div className="grid grid-cols-2 gap-3" data-testid="custom-fields-edit">
+      {fields.map((field) => {
+        const label = field.label_key ? t(field.label_key) : field.label || field.name;
+        const value = values[field.name] ?? field.value ?? field.default_value ?? '';
+
+        if (field.field_type === 'boolean') {
+          return (
+            <div key={field.name} className="flex items-center gap-2">
+              <input
+                type="checkbox"
+                id={`cf-${field.name}`}
+                checked={!!value}
+                onChange={(e) => onChange?.(field.name, e.target.checked)}
+                className="h-4 w-4 rounded border-secondary-300"
+              />
+              <label htmlFor={`cf-${field.name}`} className="text-sm text-secondary-700">
+                {label}{field.required && ' *'}
+              </label>
+            </div>
+          );
+        }
+
+        if (field.field_type === 'select') {
+          return (
+            <div key={field.name}>
+              <label htmlFor={`cf-${field.name}`} className="text-xs font-medium text-secondary-500">
+                {label}{field.required && ' *'}
+              </label>
+              <Select
+                id={`cf-${field.name}`}
+                value={value || ''}
+                options={field.options.map((opt) => ({ value: opt, label: opt }))}
+                onChange={(e) => onChange?.(field.name, e.target.value || null)}
+              />
+            </div>
+          );
+        }
+
+        if (field.field_type === 'multiselect') {
+          const selected: string[] = Array.isArray(value) ? value : [];
+          return (
+            <div key={field.name}>
+              <label htmlFor={`cf-${field.name}`} className="text-xs font-medium text-secondary-500">
+                {label}{field.required && ' *'}
+              </label>
+              <div className="flex flex-wrap gap-2 mt-1">
+                {field.options.map((opt) => (
+                  <label key={opt} className="flex items-center gap-1 text-sm">
+                    <input
+                      type="checkbox"
+                      checked={selected.includes(opt)}
+                      onChange={(e) => {
+                        if (e.target.checked) {
+                          onChange?.(field.name, [...selected, opt]);
+                        } else {
+                          onChange?.(field.name, selected.filter((v) => v !== opt));
+                        }
+                      }}
+                      className="h-4 w-4 rounded border-secondary-300"
+                    />
+                    {opt}
+                  </label>
+                ))}
+              </div>
+            </div>
+          );
+        }
+
+        // text, number, date
+        return (
+          <div key={field.name}>
+            <label htmlFor={`cf-${field.name}`} className="text-xs font-medium text-secondary-500">
+              {label}{field.required && ' *'}
+            </label>
+            <Input
+              id={`cf-${field.name}`}
+              type={field.field_type === 'number' ? 'number' : field.field_type === 'date' ? 'date' : 'text'}
+              value={value ?? ''}
+              onChange={(e) => {
+                let val: any = e.target.value;
+                if (field.field_type === 'number' && val !== '') val = Number(val);
+                if (val === '') val = null;
+                onChange?.(field.name, val);
+              }}
+            />
+          </div>
+        );
+      })}
+    </div>
+  );
+}
diff --git a/frontend/src/components/plugins/__tests__/PluginRegistry.test.tsx b/frontend/src/components/plugins/__tests__/PluginRegistry.test.tsx
index d13e39e..d2e0be7 100644
--- a/frontend/src/components/plugins/__tests__/PluginRegistry.test.tsx
+++ b/frontend/src/components/plugins/__tests__/PluginRegistry.test.tsx
@@ -21,6 +21,7 @@ const mockManifests: PluginUiManifest[] = [
     detail_tabs: [],
     settings_pages: [],
     dashboard_widgets: [],
+    custom_fields: [],
   },
 ];
 
diff --git a/frontend/src/components/plugins/__tests__/PluginRouteRenderer.test.tsx b/frontend/src/components/plugins/__tests__/PluginRouteRenderer.test.tsx
index 29b1989..07bf9d9 100644
--- a/frontend/src/components/plugins/__tests__/PluginRouteRenderer.test.tsx
+++ b/frontend/src/components/plugins/__tests__/PluginRouteRenderer.test.tsx
@@ -29,6 +29,7 @@ const mockManifests: PluginUiManifest[] = [
     detail_tabs: [],
     settings_pages: [],
     dashboard_widgets: [],
+    custom_fields: [],
   },
   {
     name: 'calendar_plugin',
@@ -42,6 +43,7 @@ const mockManifests: PluginUiManifest[] = [
     detail_tabs: [],
     settings_pages: [],
     dashboard_widgets: [],
+    custom_fields: [],
   },
 ];
 
diff --git a/frontend/src/store/__tests__/pluginStore.test.ts b/frontend/src/store/__tests__/pluginStore.test.ts
index da7a412..6b03cd6 100644
--- a/frontend/src/store/__tests__/pluginStore.test.ts
+++ b/frontend/src/store/__tests__/pluginStore.test.ts
@@ -25,6 +25,7 @@ const mockManifests: PluginUiManifest[] = [
     dashboard_widgets: [
       { id: 'widget_a', label_key: 'widgets.a', label: 'Widget A', component: '@/components/WidgetA', icon: 'A', order: 200, col_span: 2, row_span: 1, permission: '' },
     ],
+    custom_fields: [],
   },
   {
     name: 'plugin_b',
@@ -46,6 +47,7 @@ const mockManifests: PluginUiManifest[] = [
     dashboard_widgets: [
       { id: 'widget_b', label_key: 'widgets.b', label: 'Widget B', component: '@/components/WidgetB', icon: 'B', order: 100, col_span: 1, row_span: 1, permission: '' },
     ],
+    custom_fields: [],
   },
 ];
 
diff --git a/frontend/src/store/pluginStore.ts b/frontend/src/store/pluginStore.ts
index 3791330..e568360 100644
--- a/frontend/src/store/pluginStore.ts
+++ b/frontend/src/store/pluginStore.ts
@@ -50,6 +50,17 @@ export interface PluginDashboardWidget {
   permission: string;
 }
 
+export interface PluginCustomFieldDefinition {
+  name: string;
+  label: string;
+  label_key: string;
+  field_type: 'text' | 'number' | 'date' | 'select' | 'multiselect' | 'boolean';
+  options: string[];
+  default_value: any;
+  required: boolean;
+  entity: string;
+}
+
 export interface PluginUiManifest {
   name: string;
   display_name: string;
@@ -60,6 +71,7 @@ export interface PluginUiManifest {
   detail_tabs: PluginDetailTab[];
   settings_pages: PluginSettingsPage[];
   dashboard_widgets: PluginDashboardWidget[];
+  custom_fields: PluginCustomFieldDefinition[];
 }
 
 interface PluginState {
@@ -77,6 +89,7 @@ interface PluginState {
   getDetailTabsForEntity: (entityType: string) => PluginDetailTab[];
   getAllSettingsPages: () => PluginSettingsPage[];
   getAllDashboardWidgets: () => PluginDashboardWidget[];
+  getCustomFieldsForEntity: (entityType: string) => PluginCustomFieldDefinition[];
 }
 
 export const usePluginStore = create<PluginState>((set, get) => ({
@@ -125,4 +138,11 @@ export const usePluginStore = create<PluginState>((set, get) => ({
       .flatMap((m) => m.dashboard_widgets)
       .sort((a, b) => a.order - b.order);
   },
+
+  getCustomFieldsForEntity: (entityType: string) => {
+    const { manifests } = get();
+    return manifests
+      .flatMap((m) => m.custom_fields || [])
+      .filter((cf) => cf.entity === entityType);
+  },
 }));
diff --git a/tests/test_custom_fields.py b/tests/test_custom_fields.py
new file mode 100644
index 0000000..eb89587
--- /dev/null
+++ b/tests/test_custom_fields.py
@@ -0,0 +1,164 @@
+"""Custom fields tests — plugin-defined custom fields on contacts."""
+
+from __future__ import annotations
+
+import pytest
+from httpx import AsyncClient
+
+from tests.conftest import ORIGIN_HEADER, login_client, seed_tenant_and_users
+
+
+@pytest.mark.asyncio
+class TestCustomFieldsGet:
+    """GET /api/v1/contacts/{id}/custom-fields"""
+
+    async def test_get_custom_fields_returns_200(self, client: AsyncClient, db_session):
+        """GET custom fields for a contact returns 200 with merged definitions."""
+        await seed_tenant_and_users(db_session)
+        await login_client(client, "admin@tenanta.com")
+        # Create a contact first
+        resp = await client.post(
+            "/api/v1/contacts",
+            json={"type": "company", "name": "Test Corp", "displayname": "Test Corp"},
+            headers=ORIGIN_HEADER,
+        )
+        assert resp.status_code == 201
+        contact_id = resp.json()["id"]
+
+        resp = await client.get(
+            f"/api/v1/contacts/{contact_id}/custom-fields",
+            headers=ORIGIN_HEADER,
+        )
+        assert resp.status_code == 200
+        data = resp.json()
+        assert "fields" in data
+        assert isinstance(data["fields"], list)
+
+    async def test_get_custom_fields_invalid_uuid_returns_400(self, client: AsyncClient, db_session):
+        """GET custom fields with invalid UUID returns 400."""
+        await seed_tenant_and_users(db_session)
+        await login_client(client, "admin@tenanta.com")
+        resp = await client.get(
+            "/api/v1/contacts/not-a-uuid/custom-fields",
+            headers=ORIGIN_HEADER,
+        )
+        assert resp.status_code == 400
+
+    async def test_get_custom_fields_not_found_returns_404(self, client: AsyncClient, db_session):
+        """GET custom fields for non-existent contact returns 404."""
+        await seed_tenant_and_users(db_session)
+        await login_client(client, "admin@tenanta.com")
+        resp = await client.get(
+            "/api/v1/contacts/00000000-0000-0000-0000-000000000000/custom-fields",
+            headers=ORIGIN_HEADER,
+        )
+        assert resp.status_code == 404
+
+
+@pytest.mark.asyncio
+class TestCustomFieldsUpdate:
+    """PATCH /api/v1/contacts/{id}/custom-fields"""
+
+    async def test_update_custom_fields_returns_200(self, client: AsyncClient, db_session):
+        """PATCH custom fields stores values in contacts.custom JSONB."""
+        await seed_tenant_and_users(db_session)
+        await login_client(client, "admin@tenanta.com")
+        # Create a contact
+        resp = await client.post(
+            "/api/v1/contacts",
+            json={"type": "company", "name": "Custom Corp", "displayname": "Custom Corp"},
+            headers=ORIGIN_HEADER,
+        )
+        contact_id = resp.json()["id"]
+
+        # Update custom fields (empty values is valid since no plugin defines fields)
+        resp = await client.patch(
+            f"/api/v1/contacts/{contact_id}/custom-fields",
+            json={"values": {"test_field": "test_value"}},
+            headers=ORIGIN_HEADER,
+        )
+        # Without plugin definitions, unknown fields should be rejected
+        assert resp.status_code == 400
+
+    async def test_update_custom_fields_empty_values_returns_200(self, client: AsyncClient, db_session):
+        """PATCH custom fields with empty values dict returns 200."""
+        await seed_tenant_and_users(db_session)
+        await login_client(client, "admin@tenanta.com")
+        resp = await client.post(
+            "/api/v1/contacts",
+            json={"type": "company", "name": "Empty Corp", "displayname": "Empty Corp"},
+            headers=ORIGIN_HEADER,
+        )
+        contact_id = resp.json()["id"]
+
+        resp = await client.patch(
+            f"/api/v1/contacts/{contact_id}/custom-fields",
+            json={"values": {}},
+            headers=ORIGIN_HEADER,
+        )
+        assert resp.status_code == 200
+        assert "fields" in resp.json()
+
+    async def test_update_custom_fields_not_found_returns_404(self, client: AsyncClient, db_session):
+        """PATCH custom fields for non-existent contact returns 404."""
+        await seed_tenant_and_users(db_session)
+        await login_client(client, "admin@tenanta.com")
+        resp = await client.patch(
+            "/api/v1/contacts/00000000-0000-0000-0000-000000000000/custom-fields",
+            json={"values": {}},
+            headers=ORIGIN_HEADER,
+        )
+        assert resp.status_code == 404
+
+
+@pytest.mark.asyncio
+class TestCustomFieldDefinition:
+    """Test CustomFieldDefinition model in manifest."""
+
+    async def test_custom_field_definition_creation(self):
+        """CustomFieldDefinition can be created with valid field types."""
+        from app.plugins.manifest import CustomFieldDefinition
+
+        cf = CustomFieldDefinition(
+            name="lead_source",
+            label="Lead Source",
+            label_key="custom.leadSource",
+            field_type="select",
+            options=["website", "referral", "cold_call"],
+            required=False,
+            entity="contact",
+        )
+        assert cf.name == "lead_source"
+        assert cf.field_type == "select"
+        assert len(cf.options) == 3
+
+    async def test_custom_field_definition_invalid_type_raises(self):
+        """CustomFieldDefinition rejects invalid field_type."""
+        from app.plugins.manifest import CustomFieldDefinition
+        from pydantic import ValidationError
+
+        with pytest.raises(ValidationError):
+            CustomFieldDefinition(
+                name="bad_field",
+                field_type="invalid_type",
+            )
+
+    async def test_plugin_manifest_with_custom_fields(self):
+        """PluginManifest accepts custom_fields list."""
+        from app.plugins.manifest import PluginManifest, CustomFieldDefinition
+
+        manifest = PluginManifest(
+            name="test_plugin",
+            version="1.0.0",
+            display_name="Test Plugin",
+            custom_fields=[
+                CustomFieldDefinition(
+                    name="score",
+                    label="Score",
+                    field_type="number",
+                    entity="contact",
+                ),
+            ],
+        )
+        assert len(manifest.custom_fields) == 1
+        assert manifest.custom_fields[0].name == "score"