diff --git a/pxr/external/boost/python/common.hpp b/pxr/external/boost/python/common.hpp
index d0ce89fc64..5a351a29b6 100644
--- a/pxr/external/boost/python/common.hpp
+++ b/pxr/external/boost/python/common.hpp
@@ -43,12 +43,16 @@ namespace pxr_boost {
 #define PXR_BOOST_PYTHON_FUNCTION_OVERLOADS BOOST_PYTHON_FUNCTION_OVERLOADS
 
 #ifdef PXR_BOOST_PYTHON_MAX_ARITY
+#ifndef BOOST_PYTHON_MAX_ARITY
 #define BOOST_PYTHON_MAX_ARITY PXR_BOOST_PYTHON_MAX_ARITY
-#endif
+#endif // BOOST_PYTHON_MAX_ARITY
+#endif // PXR_BOOST_PYTHON_MAX_ARITY
 
 #ifdef PXR_BOOST_PYTHON_NO_PY_SIGNATURES
+#ifndef BOOST_PYTHON_NO_PY_SIGNATURES
 #define BOOST_PYTHON_NO_PY_SIGNATURES
-#endif
+#endif // BOOST_PYTHON_NO_PY_SIGNATURES
+#endif // PXR_BOOST_PYTHON_NO_PY_SIGNATURES
 
 #endif // PXR_USE_INTERNAL_BOOST_PYTHON