diff --git a/app/services/sequence_service.py b/app/services/sequence_service.py
new file mode 100644
index 0000000..924abc4
--- /dev/null
+++ b/app/services/sequence_service.py
@@ -0,0 +1,133 @@
+"""Sequence service — get_next_number with SELECT FOR UPDATE locking, CRUD, create."""
+
+from __future__ import annotations
+
+import uuid
+from datetime import UTC, datetime
+from typing import Any
+
+from sqlalchemy import select, text
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.audit import log_audit
+from app.models.sequence import Sequence
+
+
+def _sequence_to_dict(s: Sequence) -> dict[str, Any]:
+    """Serialize a Sequence ORM object to dict."""
+    return {
+        "id": str(s.id),
+        "name": s.name,
+        "prefix": s.prefix,
+        "next_number": s.next_number,
+        "padding": s.padding,
+        "created_at": s.created_at.isoformat() if s.created_at else None,
+        "updated_at": s.updated_at.isoformat() if s.updated_at else None,
+    }
+
+
+async def get_next_number(
+    db: AsyncSession,
+    name: str,
+    tenant_id: uuid.UUID,
+) -> str:
+    """Get the next formatted number for a sequence.
+
+    Uses SELECT FOR UPDATE to lock the row, increment next_number,
+    and return the formatted string (prefix + zero-padded number).
+
+    Raises ValueError if the sequence does not exist.
+    """
+    # SELECT FOR UPDATE to lock the row
+    q = (
+        select(Sequence)
+        .where(
+            Sequence.name == name,
+            Sequence.tenant_id == tenant_id,
+            Sequence.deleted_at.is_(None),
+        )
+        .with_for_update()
+    )
+    result = await db.execute(q)
+    seq = result.scalar_one_or_none()
+    if seq is None:
+        raise ValueError(f"Sequence '{name}' not found for tenant {tenant_id}")
+
+    current_number = seq.next_number
+    formatted = f"{seq.prefix}{str(current_number).zfill(seq.padding)}"
+    seq.next_number = current_number + 1
+
+    await db.flush()
+    return formatted
+
+
+async def create_sequence(
+    db: AsyncSession,
+    tenant_id: uuid.UUID,
+    user_id: uuid.UUID,
+    data: dict[str, Any],
+) -> dict[str, Any]:
+    """Create a new sequence."""
+    sequence = Sequence(
+        tenant_id=tenant_id,
+        name=data["name"],
+        prefix=data.get("prefix", ""),
+        next_number=1,
+        padding=data.get("padding", 4),
+    )
+    db.add(sequence)
+    await db.flush()
+    await db.refresh(sequence)
+    await log_audit(
+        db, tenant_id, user_id, "create", "sequence", sequence.id,
+        changes={"name": sequence.name, "prefix": sequence.prefix},
+    )
+    return _sequence_to_dict(sequence)
+
+
+async def list_sequences(
+    db: AsyncSession,
+    tenant_id: uuid.UUID,
+) -> dict[str, Any]:
+    """List all sequences for a tenant."""
+    q = select(Sequence).where(
+        Sequence.tenant_id == tenant_id,
+        Sequence.deleted_at.is_(None),
+    ).order_by(Sequence.name.asc())
+    result = await db.execute(q)
+    sequences = result.scalars().all()
+    return {
+        "items": [_sequence_to_dict(s) for s in sequences],
+        "total": len(sequences),
+    }
+
+
+async def update_sequence(
+    db: AsyncSession,
+    tenant_id: uuid.UUID,
+    user_id: uuid.UUID,
+    sequence_id: uuid.UUID,
+    data: dict[str, Any],
+) -> dict[str, Any] | None:
+    """Update a sequence (name, prefix, padding only — NOT next_number)."""
+    q = select(Sequence).where(
+        Sequence.id == sequence_id,
+        Sequence.tenant_id == tenant_id,
+        Sequence.deleted_at.is_(None),
+    )
+    result = await db.execute(q)
+    sequence = result.scalar_one_or_none()
+    if sequence is None:
+        return None
+
+    changes: dict[str, Any] = {}
+    for field in ("name", "prefix", "padding"):
+        if field in data and data[field] is not None:
+            old_val = getattr(sequence, field)
+            changes[field] = {"old": old_val, "new": data[field]}
+            setattr(sequence, field, data[field])
+
+    await db.flush()
+    await db.refresh(sequence)
+    await log_audit(db, tenant_id, user_id, "update", "sequence", sequence_id, changes=changes)
+    return _sequence_to_dict(sequence)