diff --git a/app/sync/manager.py b/app/sync/manager.py
index b4e5e6c..78b37fc 100644
--- a/app/sync/manager.py
+++ b/app/sync/manager.py
@@ -17,12 +17,18 @@ from app.sync.states import SyncOutcome
 logger = logging.getLogger(__name__)
 
 
+class SyncAlreadyRunning(RuntimeError):
+    pass
+
+
 class SyncManager:
     """Resumable single-user MyWhoosh -> Garmin sync pipeline.
 
-    Locking/scheduling across multiple users is layered on top of `sync_user`
-    by a later task; this class only implements the state machine for one
-    user's sync run.
+    `_sync_user_locked` implements the state machine for one user's sync run.
+    `sync_user` wraps it with a per-user `asyncio.Lock` so only one sync can
+    run for a given user at a time (raising `SyncAlreadyRunning` on overlap),
+    and `sync_all_enabled` fans out across all enabled users, isolating each
+    user's failure to its own result rather than cancelling siblings.
     """
 
     def __init__(
@@ -41,8 +47,32 @@ class SyncManager:
         self.mywhoosh_factory = mywhoosh_factory
         self.garmin_factory = garmin_factory
         self.fit_converter = fit_converter
+        self._locks: dict[int, asyncio.Lock] = {}
+        self._locks_guard = asyncio.Lock()
+
+    async def _lock_for(self, user_id: int) -> asyncio.Lock:
+        async with self._locks_guard:
+            return self._locks.setdefault(user_id, asyncio.Lock())
 
     async def sync_user(self, user_id: int, mfa_code: str | None = None) -> SyncOutcome:
+        lock = await self._lock_for(user_id)
+        if lock.locked():
+            raise SyncAlreadyRunning(f"sync already running for user {user_id}")
+        async with lock:
+            return await self._sync_user_locked(user_id, mfa_code)
+
+    def _load_enabled_user_ids(self) -> list[int]:
+        with self.session_factory() as session:
+            return [user.id for user in UserRepository(session).list_enabled()]
+
+    async def sync_all_enabled(self) -> list[SyncOutcome | Exception]:
+        user_ids = self._load_enabled_user_ids()
+        return await asyncio.gather(
+            *(self.sync_user(user_id) for user_id in user_ids),
+            return_exceptions=True,
+        )
+
+    async def _sync_user_locked(self, user_id: int, mfa_code: str | None = None) -> SyncOutcome:
         with self.session_factory() as session:
             user = UserRepository(session).get(user_id)
             if user is None:
diff --git a/tests/sync/test_concurrency.py b/tests/sync/test_concurrency.py
new file mode 100644
index 0000000..cfcc01c
--- /dev/null
+++ b/tests/sync/test_concurrency.py
@@ -0,0 +1,163 @@
+import asyncio
+
+import pytest
+
+from app.db.repositories import UserRepository
+from app.mywhoosh.client import MyWhooshAuthError
+from app.mywhoosh.models import MyWhooshActivity
+from app.sync.manager import SyncAlreadyRunning, SyncManager
+from tests.sync.conftest import FakeFitConverter, _create_user
+from tests.sync.fakes import FakeGarminUploader
+
+
+class BlockingMyWhooshClient:
+    """Fake MyWhoosh client whose list_activities() blocks on test-controlled
+    events, so a test can deterministically observe "sync has started but not
+    finished" without any production-only test hooks."""
+
+    def __init__(self, first_started: asyncio.Event, release: asyncio.Event) -> None:
+        self.first_started = first_started
+        self.release = release
+        self.list_calls = 0
+
+    async def list_activities(self, email: str, password: str):
+        self.list_calls += 1
+        self.first_started.set()
+        await self.release.wait()
+        return []
+
+    async def download_fit(self, activity_file_id: str, email: str, password: str) -> bytes:
+        raise AssertionError("download_fit should not be reached in this test")
+
+
+class ConditionalFailureMyWhooshClient:
+    """Fake MyWhoosh client that raises MyWhooshAuthError only for a specific
+    account email, letting one user's sync fail while others succeed."""
+
+    def __init__(self, activities, fit_bytes: bytes, failing_email: str) -> None:
+        self.activities = activities
+        self.fit_bytes = fit_bytes
+        self.failing_email = failing_email
+        self.list_calls = 0
+        self.download_calls = 0
+
+    async def list_activities(self, email: str, password: str):
+        self.list_calls += 1
+        if email == self.failing_email:
+            raise MyWhooshAuthError("simulated auth failure")
+        return list(self.activities)
+
+    async def download_fit(self, activity_file_id: str, email: str, password: str) -> bytes:
+        self.download_calls += 1
+        return self.fit_bytes
+
+
+@pytest.fixture
+def user_a(session_factory, cipher):
+    with session_factory() as session:
+        return _create_user(session, cipher)
+
+
+@pytest.fixture
+def user_b(session_factory, cipher):
+    with session_factory() as session:
+        return _create_user(session, cipher)
+
+
+@pytest.mark.asyncio
+async def test_same_user_cannot_run_twice(session_factory, cipher, settings, seeded_user) -> None:
+    first_started = asyncio.Event()
+    release_first = asyncio.Event()
+    mywhoosh = BlockingMyWhooshClient(first_started, release_first)
+    manager = SyncManager(
+        session_factory=session_factory,
+        credential_cipher=cipher,
+        settings=settings,
+        mywhoosh_factory=lambda token_store: mywhoosh,
+        garmin_factory=lambda email, password, tokenstore: FakeGarminUploader(),
+        fit_converter=FakeFitConverter(),
+    )
+
+    first = asyncio.create_task(manager.sync_user(seeded_user.id))
+    await first_started.wait()
+
+    with pytest.raises(SyncAlreadyRunning):
+        await manager.sync_user(seeded_user.id)
+
+    release_first.set()
+    outcome = await first
+
+    assert outcome.user_id == seeded_user.id
+    assert mywhoosh.list_calls == 1
+
+
+@pytest.mark.asyncio
+async def test_different_users_can_run_concurrently(manager, user_a, user_b) -> None:
+    results = await asyncio.gather(manager.sync_user(user_a.id), manager.sync_user(user_b.id))
+    assert {result.user_id for result in results} == {user_a.id, user_b.id}
+    assert all(result.status == "success" for result in results)
+
+
+@pytest.mark.asyncio
+async def test_sync_all_enabled_isolates_failures(session_factory, cipher, settings) -> None:
+    with session_factory() as session:
+        failing_user = UserRepository(session).create(
+            name="Failing User",
+            enabled=True,
+            mywhoosh_email_enc=cipher.encrypt("failing-mw@example.com"),
+            mywhoosh_password_enc=cipher.encrypt("failing-mw-pass"),
+            garmin_email_enc=cipher.encrypt("failing-garmin@example.com"),
+            garmin_password_enc=cipher.encrypt("failing-garmin-pass"),
+        )
+        healthy_user = UserRepository(session).create(
+            name="Healthy User",
+            enabled=True,
+            mywhoosh_email_enc=cipher.encrypt("healthy-mw@example.com"),
+            mywhoosh_password_enc=cipher.encrypt("healthy-mw-pass"),
+            garmin_email_enc=cipher.encrypt("healthy-garmin@example.com"),
+            garmin_password_enc=cipher.encrypt("healthy-garmin-pass"),
+        )
+
+    remote = MyWhooshActivity(
+        id="mw-shared",
+        title="Shared Ride",
+        activity_file_id="file-mw-shared",
+        started_at=None,
+    )
+    mywhoosh = ConditionalFailureMyWhooshClient(
+        activities=[remote],
+        fit_bytes=b"source-bytes",
+        failing_email="failing-mw@example.com",
+    )
+    converter = FakeFitConverter()
+    garmin = FakeGarminUploader()
+    manager = SyncManager(
+        session_factory=session_factory,
+        credential_cipher=cipher,
+        settings=settings,
+        mywhoosh_factory=lambda token_store: mywhoosh,
+        garmin_factory=lambda email, password, tokenstore: garmin,
+        fit_converter=converter,
+    )
+
+    results = await manager.sync_all_enabled()
+
+    assert len(results) == 2
+    for result in results:
+        assert not isinstance(result, Exception)
+
+    by_user = {result.user_id: result for result in results}
+    failing_outcome = by_user[failing_user.id]
+    healthy_outcome = by_user[healthy_user.id]
+
+    # The failing user's auth error is classified by _sync_user_locked's own
+    # exception handling and returned as a non-success SyncOutcome rather than
+    # raised -- so asyncio.gather never sees an exception for this failure
+    # mode. It must not affect the healthy user's independent outcome.
+    assert failing_outcome.status != "success"
+    assert failing_outcome.message is not None
+    assert failing_outcome.discovered == 0
+
+    assert healthy_outcome.status == "success"
+    assert healthy_outcome.imported == 1
+    assert healthy_outcome.failed == 0