diff --git a/airflow-core/src/airflow/serialization/definitions/taskgroup.py b/airflow-core/src/airflow/serialization/definitions/taskgroup.py
index 65d59cb15f17e..1b353b4c7d910 100644
--- a/airflow-core/src/airflow/serialization/definitions/taskgroup.py
+++ b/airflow-core/src/airflow/serialization/definitions/taskgroup.py
@@ -251,6 +251,11 @@ def _project_child_deps(
         self, child_idx: int, child: DAGNode, id_to_idx: dict[str, int]
     ) -> tuple[int, ...]:
         upstream_ids = child.upstream_task_ids
+        if isinstance(child, SerializedTaskGroup):
+            root_upstream_ids = set()
+            for root_task in child.get_roots():
+                root_upstream_ids.update(root_task.upstream_task_ids)
+            upstream_ids = upstream_ids | root_upstream_ids
         if not upstream_ids:
             return ()
         sib_deps: set[int] = set()
diff --git a/airflow-core/tests/unit/utils/test_task_group.py b/airflow-core/tests/unit/utils/test_task_group.py
index 2d1458e95fbd1..d3708bbde0a6c 100644
--- a/airflow-core/tests/unit/utils/test_task_group.py
+++ b/airflow-core/tests/unit/utils/test_task_group.py
@@ -1150,6 +1150,31 @@ def nested_topo(group):
     ]
 
 
+def test_topological_sort_serialized_task_level_cross_group_dep():
+    """Task-level deps between groups are respected for ordering after serialization.
+
+    When a task inside group_b depends on a task inside group_a, the serialized
+    topological sort must place group_a before group_b.
+    """
+    with DAG("test_cross_group_task_dep", schedule=None, start_date=DEFAULT_DATE) as dag:
+        with TaskGroup("stage_b"):
+            b_start = EmptyOperator(task_id="b_start")
+            b_end = EmptyOperator(task_id="b_end")
+            b_start >> b_end
+
+        with TaskGroup("stage_a"):
+            a_start = EmptyOperator(task_id="a_start")
+            a_end = EmptyOperator(task_id="a_end")
+            a_start >> a_end
+
+        b_end >> a_start
+
+    serialized = create_scheduler_dag(dag)
+    order = [node.node_id for node in serialized.task_group.topological_sort()]
+
+    assert order.index("stage_b") < order.index("stage_a")
+
+
 def test_topological_sort_serialized_layered():
     """SerializedTaskGroup.topological_sort emits a valid order after DAG round-trip.