diff --git a/xcp_d/cli/run.py b/xcp_d/cli/run.py
index 4a814bd70..8f3a72e41 100644
--- a/xcp_d/cli/run.py
+++ b/xcp_d/cli/run.py
@@ -745,6 +745,16 @@ def _validate_parameters(opts, build_log):
         else:
             build_log.error(f"Freesurfer license DNE: {opts.fs_license_file}.")
             return_code = 1
+    else:
+        fs_license_file = os.environ.get("FS_LICENSE", "/opt/freesurfer/license.txt")
+        if not Path(fs_license_file).is_file():
+            build_log.error(
+                "A valid FreeSurfer license file is required. "
+                "Set the FS_LICENSE environment variable or use the '--fs-license-file' flag."
+            )
+            return_code = 1
+
+        os.environ["FS_LICENSE"] = str(fs_license_file)
 
     # Check the validity of inputs
     if opts.output_dir == opts.fmri_dir: