diff --git a/.changeset/fix-join-on-demand-sync.md b/.changeset/fix-join-on-demand-sync.md
new file mode 100644
index 000000000..163062d62
--- /dev/null
+++ b/.changeset/fix-join-on-demand-sync.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/db': patch
+---
+
+Fix on-demand sync collections not loading data when used in join queries. Previously, collections with `syncMode: 'on-demand'` would remain idle when used as sources in join queries, causing empty results. Now `startSyncImmediate()` is called on all source collections in `subscribeToAllCollections()` to ensure sync is properly initialized.
diff --git a/packages/db/src/query/live/collection-config-builder.ts b/packages/db/src/query/live/collection-config-builder.ts
index b663fdad5..1706be42c 100644
--- a/packages/db/src/query/live/collection-config-builder.ts
+++ b/packages/db/src/query/live/collection-config-builder.ts
@@ -866,6 +866,11 @@ export class CollectionConfigBuilder<
       })
       syncState.unsubscribeCallbacks.add(statusUnsubscribe)
 
+      // Ensure sync is started for all source collections in the query.
+      // This is critical for on-demand sync collections used in joins - without this,
+      // they remain idle and requestSnapshot() calls from lazy join loading won't work.
+      collection.startSyncImmediate()
+
       const subscription = collectionSubscriber.subscribe()
       // Store subscription by alias (not collection ID) to support lazy loading
       // which needs to look up subscriptions by their query alias
diff --git a/packages/db/src/query/live/collection-subscriber.ts b/packages/db/src/query/live/collection-subscriber.ts
index 303c833fc..c56e610c2 100644
--- a/packages/db/src/query/live/collection-subscriber.ts
+++ b/packages/db/src/query/live/collection-subscriber.ts
@@ -197,6 +197,12 @@ export class CollectionSubscriber<
       whereExpression,
     })
 
+    // Trigger initial data load for on-demand sync collections.
+    // This is called regardless of includeInitialState because on-demand collections
+    // need to explicitly request data loading. For lazy aliases in joins,
+    // includeInitialState is false but we still need to trigger the sync layer.
+    subscription.requestSnapshot({})
+
     return subscription
   }