diff --git a/hexrdgui/main.py b/hexrdgui/main.py
index 9b9fb707e..2f6ee0cde 100644
--- a/hexrdgui/main.py
+++ b/hexrdgui/main.py
@@ -14,6 +14,18 @@
     # Prevent crashing when using OpenBLAS
     os.environ['OPENBLAS_NUM_THREADS'] = '1'
 
+if sys.platform in ('linux', 'darwin'):
+    # Use OpenMP instead of TBB as the numba threading layer.
+    # TBB prints an unavoidable C-level warning to stderr on every
+    # non-main-thread fork ("Attempted to fork from a non-main thread"),
+    # which is harmless but noisy in our Qt background thread workflow.
+    # OpenMP is equally thread-safe and performant, emits no fork
+    # warning, and has a built-in safety net: it terminates any forked
+    # child that accidentally calls a numba parallel=True function,
+    # rather than silently running with undefined behavior.
+    # workqueue is not viable (crashes on concurrent thread access).
+    os.environ.setdefault('NUMBA_THREADING_LAYER', 'omp')
+
 from PySide6.QtCore import QCoreApplication, Qt
 from PySide6.QtGui import QIcon, QPixmap
 from PySide6.QtWidgets import QApplication