diff --git a/drift/CHANGELOG.md b/drift/CHANGELOG.md
index d26092227..5e16bc27b 100644
--- a/drift/CHANGELOG.md
+++ b/drift/CHANGELOG.md
@@ -1,3 +1,7 @@
+## Unreleased
+
+- Web: Fix nested transactions failing to commit and hanging on rollback with `WasmStorageImplementation.opfsLocks`. A nested transaction completed the shared navigator-lock completer, so the outer commit failed with `Bad state: Future already completed` and its rollback never resolved (#3870).
+
 ## 2.35.1
 
 - Web: Properly release transactions held by closing tabs.
diff --git a/drift/lib/src/web/wasm_setup/navigator_locks_interceptor.dart b/drift/lib/src/web/wasm_setup/navigator_locks_interceptor.dart
index 1ea4cf0d1..8134d49a2 100644
--- a/drift/lib/src/web/wasm_setup/navigator_locks_interceptor.dart
+++ b/drift/lib/src/web/wasm_setup/navigator_locks_interceptor.dart
@@ -19,15 +19,17 @@ final class NavigatorLocksExecutor implements QueryExecutor {
 
   @override
   QueryExecutor beginExclusive() {
-    return _inner.beginExclusive().interceptWith(
-      _AcquireNavigatorLockInterceptor(this),
+    final inner = _inner.beginExclusive();
+    return inner.interceptWith(
+      _AcquireNavigatorLockInterceptor(this, ownsLock: inner),
     );
   }
 
   @override
   TransactionExecutor beginTransaction() {
-    return _inner.beginTransaction().interceptWith(
-          _AcquireNavigatorLockInterceptor(this),
+    final inner = _inner.beginTransaction();
+    return inner.interceptWith(
+          _AcquireNavigatorLockInterceptor(this, ownsLock: inner),
         )
         as TransactionExecutor;
   }
@@ -103,9 +105,18 @@ final class _AcquireNavigatorLockInterceptor extends QueryInterceptor {
   final NavigatorLocksExecutor _executor;
   final Completer<void> _returnNavigatorLocks = Completer();
 
+  /// The executor whose commit, rollback or close ends this interceptor's
+  /// navigator-lock scope. Executors for nested transactions flow through
+  /// this same interceptor but must not complete the shared completer.
+  /// See https://github.com/simolus3/drift/issues/3870
+  final QueryExecutor _ownsNavigatorLock;
+
   Future<void>? _acquiredNavigatorLock;
 
-  _AcquireNavigatorLockInterceptor(this._executor);
+  _AcquireNavigatorLockInterceptor(
+    this._executor, {
+    required QueryExecutor ownsLock,
+  }) : _ownsNavigatorLock = ownsLock;
 
   @override
   Future<bool> ensureOpen(
@@ -122,18 +133,25 @@ final class _AcquireNavigatorLockInterceptor extends QueryInterceptor {
 
   @override
   Future<void> close(QueryExecutor inner) {
-    return inner.close().whenComplete(() => _returnNavigatorLocks.complete());
+    return inner.close().whenComplete(() => _releaseLock(inner));
   }
 
   @override
   Future<void> commitTransaction(TransactionExecutor inner) {
-    return inner.send().whenComplete(() => _returnNavigatorLocks.complete());
+    return inner.send().whenComplete(() => _releaseLock(inner));
   }
 
   @override
   Future<void> rollbackTransaction(TransactionExecutor inner) {
-    return inner.rollback().whenComplete(
-      () => _returnNavigatorLocks.complete(),
-    );
+    return inner.rollback().whenComplete(() => _releaseLock(inner));
+  }
+
+  void _releaseLock(QueryExecutor inner) {
+    // A nested transaction completes before the outermost one; releasing the
+    // navigator lock here would make the outer commit fail with
+    // `Bad state: Future already completed` and its rollback hang forever.
+    if (identical(inner, _ownsNavigatorLock)) {
+      _returnNavigatorLocks.complete();
+    }
   }
 }
diff --git a/drift/test/platforms/web/navigator_locks_interceptor_test.dart b/drift/test/platforms/web/navigator_locks_interceptor_test.dart
index 2a42c213e..8dcceaf58 100644
--- a/drift/test/platforms/web/navigator_locks_interceptor_test.dart
+++ b/drift/test/platforms/web/navigator_locks_interceptor_test.dart
@@ -40,6 +40,37 @@ void main() {
     expect(await db.todosTable.all().get(), isNotEmpty);
   });
 
+  test('nested transactions complete the outer transaction', () async {
+    // Regression test for https://github.com/simolus3/drift/issues/3870:
+    // the nested transaction used to complete the shared navigator-lock
+    // completer, so the outer commit failed and its rollback hung forever.
+    final db = TodoDb(connect());
+    addTearDown(db.close);
+
+    await db.transaction(() async {
+      await db.transaction(() async {});
+    });
+  });
+
+  test('nested transaction with rollback rolls the outer one back', () async {
+    final db = TodoDb(connect());
+    addTearDown(db.close);
+
+    final outer = db.transaction<void>(() async {
+      await db
+          .transaction(() async {
+            await db.todosTable.insertOne(
+              TodosTableCompanion.insert(content: 'nested'),
+            );
+          })
+          .then((_) => throw Exception('rollback after nested commit'));
+    });
+    // The nested commit succeeded, but the outer transaction rolls back.
+    await expectLater(outer, throwsA(isA<Exception>()));
+
+    expect(await db.todosTable.all().get(), isEmpty);
+  });
+
   test("can't have concurrent transactions", () async {
     var concurrentTransactions = 0;
     var totalTransactions = 0;