diff --git a/returnn/tf/layers/basic.py b/returnn/tf/layers/basic.py
index 390a555a74..b31cb84504 100644
--- a/returnn/tf/layers/basic.py
+++ b/returnn/tf/layers/basic.py
@@ -5521,12 +5521,14 @@ class StackLayer(LayerBase):
   """
   layer_class = "stack"
 
-  def __init__(self, axis=None, **kwargs):
+  def __init__(self, axis=None, out_spatial_dim=None, **kwargs):
     """
     :param int|None axis: new axis.
       If not given, will use Data.get_default_new_axis_for_dim_tag(<spatial>),
       i.e. some reasonable default for a new spatial axis.
+    :param DimensionTag|None out_spatial_dim:
     """
+    out_spatial_dim  # noqa  # handled in get_out_data_from_opts
     super(StackLayer, self).__init__(**kwargs)
     axis_, common_source = self._get_axis_and_common(self.sources)
     if axis is None:
@@ -5543,24 +5545,28 @@ def _get_axis_and_common(cls, sources):
     :param list[LayerBase] sources:
     :rtype: (int,Data)
     """
-    from returnn.tf.util.basic import DimensionTag
     common_source = Data.get_common_data([src.output for src in sources]).copy_template()
-    tag = DimensionTag(kind=DimensionTag.Types.Spatial, dimension=1)
-    return common_source.get_default_new_axis_for_dim_tag(tag), common_source
+    dummy_tag = DimensionTag(kind=DimensionTag.Types.Spatial, dimension=1)
+    return common_source.get_default_new_axis_for_dim_tag(dummy_tag), common_source
 
   @classmethod
-  def get_out_data_from_opts(cls, name, sources, axis=None, **kwargs):
+  def get_out_data_from_opts(cls, name, sources, axis=None, out_spatial_dim=None, **kwargs):
     """
     :param str name:
     :param list[LayerBase] sources:
     :param int|None axis:
+    :param DimensionTag|None out_spatial_dim:
     :rtype: Data
     """
     axis_, common_source = cls._get_axis_and_common(sources)
     if axis is None:
       axis = axis_
     out = common_source.copy_template(name="%s_output" % name)
-    out = out.copy_add_spatial_dim(spatial_dim_axis=axis, dim=len(sources))
+    if not out_spatial_dim:
+      out_spatial_dim = DimensionTag(
+        kind=DimensionTag.Types.Spatial, description="%s:stack" % name, dimension=len(sources))
+    assert out_spatial_dim.dimension == len(sources)
+    out = out.copy_add_dim_by_tag(axis=axis, dim_tag=out_spatial_dim, unbroadcast=True)
     return out