diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0d93203..031088a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -196,6 +196,7 @@ if(NOT TARGET pybind11_headers)
 
   target_compile_features(pybind11_headers INTERFACE cxx_inheriting_constructors cxx_user_literals
                                                      cxx_right_angle_brackets)
+  target_compile_definitions (pybind11_headers INTERFACE $<$<CONFIG:Debug>:Py_DEBUG>)
   if(NOT "${PYBIND11_INTERNALS_VERSION}" STREQUAL "")
     target_compile_definitions(
       pybind11_headers INTERFACE "PYBIND11_INTERNALS_VERSION=${PYBIND11_INTERNALS_VERSION}")