diff --git a/backend/app/core/tenant_context.py b/backend/app/core/tenant_context.py
new file mode 100644
index 00000000..9cdb80c2
--- /dev/null
+++ b/backend/app/core/tenant_context.py
@@ -0,0 +1,92 @@
+# backend/app/core/tenant_context.py
+"""
+Per-request tenant context for row-level security.
+
+Flow:
+  1. require_tenant_context (FastAPI dep) calls set_current_account_id().
+  2. The SQLAlchemy transaction-begin listener fires on every new transaction
+     and calls set_config('app.current_account_id', <id>, true) automatically.
+  3. PostgreSQL RLS policies read current_setting('app.current_account_id', TRUE)
+     to filter rows.
+
+The ContextVar is asyncio-task-scoped: each concurrent request has its own value.
+set_config with is_local=true is transaction-scoped: it resets on COMMIT or
+ROLLBACK, so the listener re-applies it at the start of every transaction.
+"""
+import contextvars
+from typing import TYPE_CHECKING
+from uuid import UUID
+
+from sqlalchemy import event, or_, text
+from sqlalchemy.ext.asyncio import AsyncEngine
+
+if TYPE_CHECKING:
+    from app.models.user import User
+
+# One slot per async task — each concurrent request gets its own value.
+_current_account_id: contextvars.ContextVar[str | None] = contextvars.ContextVar(
+    "current_account_id", default=None
+)
+
+# Platform account — global content visible to all tenants.
+PLATFORM_ACCOUNT_ID = UUID("00000000-0000-0000-0000-000000000001")
+
+
+def set_current_account_id(account_id: UUID) -> contextvars.Token:
+    """Set tenant context for the current request coroutine.
+
+    Returns a token so the caller can reset it after the request.
+    """
+    return _current_account_id.set(str(account_id))
+
+
+def clear_current_account_id(token: contextvars.Token) -> None:
+    """Reset the ContextVar to its previous value (call in finally block)."""
+    _current_account_id.reset(token)
+
+
+def get_current_account_id() -> str | None:
+    """Return the account_id string for the current request, or None."""
+    return _current_account_id.get()
+
+
+def register_tenant_listener(engine: AsyncEngine) -> None:
+    """Register the transaction-begin listener on the given engine.
+
+    Must be called once at application startup, AFTER the engine is created.
+    The listener issues set_config() at the start of every transaction so that
+    the setting is re-applied automatically even when a request commits
+    mid-flight and starts a new transaction.
+
+    Do NOT call this on admin_engine — admin connections must never set tenant
+    context automatically.
+    """
+
+    @event.listens_for(engine.sync_engine, "begin")
+    def _on_transaction_begin(conn) -> None:  # noqa: ANN001
+        account_id = _current_account_id.get()
+        if account_id:
+            # set_config(name, value, is_local=true) ≡ SET LOCAL.
+            # Unlike SET LOCAL, set_config IS parameterisable.
+            conn.execute(
+                text("SELECT set_config('app.current_account_id', :id, true)"),
+                {"id": account_id},
+            )
+        # If no account_id is set, do nothing. The RLS policy falls back to a
+        # null-matching UUID and returns zero rows — fail-closed behaviour.
+
+
+def tenant_filter(Model, current_user: "User"):  # noqa: ANN001
+    """SQLAlchemy filter clause for tables that contain platform-owned rows.
+
+    Use for: tree_tags, tree_categories, step_categories, step_library,
+             template_trees, platform_steps.
+
+    For tenant-only tables (trees, sessions, psa_connections, etc.) use:
+        Model.account_id == current_user.account_id
+    directly.
+    """
+    return or_(
+        Model.account_id == current_user.account_id,
+        Model.account_id == PLATFORM_ACCOUNT_ID,
+    )
diff --git a/backend/tests/test_tenant_context.py b/backend/tests/test_tenant_context.py
new file mode 100644
index 00000000..f3a2e89b
--- /dev/null
+++ b/backend/tests/test_tenant_context.py
@@ -0,0 +1,43 @@
+import asyncio
+from uuid import UUID
+import pytest
+from unittest.mock import MagicMock
+
+from app.core.tenant_context import (
+    set_current_account_id,
+    clear_current_account_id,
+    get_current_account_id,
+)
+
+
+def test_contextvar_is_none_by_default():
+    assert get_current_account_id() is None
+
+
+def test_set_and_clear():
+    account_id = UUID("aaaaaaaa-0000-0000-0000-000000000001")
+    token = set_current_account_id(account_id)
+    assert get_current_account_id() == str(account_id)
+    clear_current_account_id(token)
+    assert get_current_account_id() is None
+
+
+def test_tasks_are_isolated():
+    """Each asyncio task has its own ContextVar value."""
+    results = {}
+
+    async def set_in_task(name: str, value: str):
+        token = set_current_account_id(UUID(value))
+        await asyncio.sleep(0)
+        results[name] = get_current_account_id()
+        clear_current_account_id(token)
+
+    async def run():
+        await asyncio.gather(
+            set_in_task("a", "aaaaaaaa-0000-0000-0000-000000000001"),
+            set_in_task("b", "bbbbbbbb-0000-0000-0000-000000000002"),
+        )
+
+    asyncio.run(run())
+    assert results["a"] == "aaaaaaaa-0000-0000-0000-000000000001"
+    assert results["b"] == "bbbbbbbb-0000-0000-0000-000000000002"