diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index c457e149c1..bc9c2dd1f8 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -9,6 +9,15 @@
 */
 
 #pragma once
+
+// See Issue #5956. This fixes compilation failure with MSVC 17.12 (v14.44) and C++20:
+// '_BACKUP_ITERATOR_DEBUG_LEVEL' undeclared when including span header from STL.
+#if defined(_MSC_VER)
+#    ifndef _BACKUP_ITERATOR_DEBUG_LEVEL
+#        define _BACKUP_ITERATOR_DEBUG_LEVEL 0
+#    endif
+#endif
+
 #include "detail/class.h"
 #include "detail/dynamic_raw_ptr_cast_if_possible.h"
 #include "detail/exception_translation.h"