diff --git a/app/schemas/activity.py b/app/schemas/activity.py
new file mode 100644
index 0000000..4976d0c
--- /dev/null
+++ b/app/schemas/activity.py
@@ -0,0 +1,68 @@
+"""Pydantic schemas for Activity endpoints."""
+
+from __future__ import annotations
+
+from datetime import datetime
+from typing import Optional
+
+from pydantic import BaseModel, ConfigDict, Field, model_validator
+
+from app.models.activity import ActivityType
+
+
+class ActivityBase(BaseModel):
+    type: ActivityType
+    subject: str = Field(..., min_length=1, max_length=255)
+    body: Optional[str] = None
+    due_date: Optional[datetime] = None
+    account_id: Optional[int] = None
+    contact_id: Optional[int] = None
+    deal_id: Optional[int] = None
+
+    @model_validator(mode="after")
+    def _check_at_least_one_parent(self) -> "ActivityBase":
+        if self.account_id is None and self.contact_id is None and self.deal_id is None:
+            raise ValueError(
+                "At least one of account_id, contact_id, deal_id must be set"
+            )
+        return self
+
+
+class ActivityCreate(ActivityBase):
+    pass
+
+
+class ActivityUpdate(BaseModel):
+    type: Optional[ActivityType] = None
+    subject: Optional[str] = Field(None, min_length=1, max_length=255)
+    body: Optional[str] = None
+    due_date: Optional[datetime] = None
+    account_id: Optional[int] = None
+    contact_id: Optional[int] = None
+    deal_id: Optional[int] = None
+
+
+class ActivityOut(ActivityBase):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    org_id: int
+    owner_id: int
+    completed_at: Optional[datetime] = None
+    created_at: datetime
+    updated_at: datetime
+    deleted_at: Optional[datetime] = None
+
+
+class ActivityCompleteRequest(BaseModel):
+    """Body for PATCH /api/v1/activities/{id}/complete."""
+
+    outcome: Optional[str] = Field(None, max_length=2048)
+
+
+__all__ = [
+    "ActivityCompleteRequest",
+    "ActivityCreate",
+    "ActivityOut",
+    "ActivityUpdate",
+]