diff --git a/python/lsst/daf/butler/dimensions/_universe.py b/python/lsst/daf/butler/dimensions/_universe.py
index 0d98a2936a..db7fdc9c32 100644
--- a/python/lsst/daf/butler/dimensions/_universe.py
+++ b/python/lsst/daf/butler/dimensions/_universe.py
@@ -418,6 +418,12 @@ def getDimensionIndex(self, name: str) -> int:
         """
         return self._dimensionIndices[name]
 
+    # TODO: remove on DM-41326.
+    @deprecated(
+        "Deprecated in favor of DimensionUniverse.conform, and will be removed after v27.",
+        version="v27",
+        category=FutureWarning,
+    )
     def expandDimensionNameSet(self, names: set[str]) -> None:
         """Expand a set of dimension names in-place.