diff --git a/apis/python/src/tiledbsoma/_query.py b/apis/python/src/tiledbsoma/_query.py
index 3ee8e40f7f..c3d36b0a6f 100644
--- a/apis/python/src/tiledbsoma/_query.py
+++ b/apis/python/src/tiledbsoma/_query.py
@@ -752,6 +752,7 @@ def to_spatialdata(  # type: ignore[no-untyped-def]
                         "Unable to export to SpatialData; exported assets have "
                         "overlapping observations."
                     ) from err
+                ad.obs["region_key"] = pd.Categorical(ad.obs["region_key"])
                 regions = list(region_joinids.keys())
                 region_key = "region_key"
                 instance_key = "instance_key"