diff --git a/mne/bem.py b/mne/bem.py
index bb73544f763..221569ee817 100644
--- a/mne/bem.py
+++ b/mne/bem.py
@@ -1313,7 +1313,18 @@ def make_watershed_bem(
         f"\nResults dir = {ws_dir}\nCommand = {' '.join(cmd)}\n"
     )
     os.makedirs(op.join(ws_dir))
-    run_subprocess_env(cmd)
+    try:
+        run_subprocess_env(cmd)
+    except FileNotFoundError:
+        raise RuntimeError(
+            "FreeSurfer executable 'mri_watershed' not found.\n\n"
+            "This usually means FreeSurfer is not properly configured.\n"
+            "Make sure:\n"
+            "- FREESURFER_HOME is set\n"
+            "- $FREESURFER_HOME/bin is in your PATH\n"
+            "- You started Python/Jupyter from a terminal where SetupFreeSurfer.sh is sourced\n\n"
+            "See MNE installation documentation for details."
+        )
     del tempdir  # clean up directory
     if op.isfile(T1_mgz):
         new_info = _extract_volume_info(T1_mgz)