diff --git a/doc/changelog.dox b/doc/changelog.dox
index 3c430486a..5abaf9705 100644
--- a/doc/changelog.dox
+++ b/doc/changelog.dox
@@ -264,6 +264,9 @@ See also:
     for Emscripten (see [mosra/magnum#219](https://github.com/mosra/magnum/issues/219))
 -   Updated the Vcpkg package to work correctly with Vcpkg's own SDL2 (see
     [Microsoft/vcpkg#5730](https://github.com/Microsoft/vcpkg/pull/5730))
+-   Updated bundled GL headers to properly define the `GL_VERSION_*` macros to
+    avoid conflicting `GLintptr` and `GLsizeiptr` type definitions on Windows
+    when using Magnum together with Qt
 
 @subsection changelog-latest-bugfixes Bug fixes
 
diff --git a/src/MagnumExternal/OpenGL/GL/flextGL.h b/src/MagnumExternal/OpenGL/GL/flextGL.h
index cdc1b6ae2..7d36f26e3 100644
--- a/src/MagnumExternal/OpenGL/GL/flextGL.h
+++ b/src/MagnumExternal/OpenGL/GL/flextGL.h
@@ -93,6 +93,29 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+#define GL_VERSION_1_0 1
+#define GL_VERSION_1_1 1
+#define GL_VERSION_1_2 1
+#define GL_VERSION_1_3 1
+#define GL_VERSION_1_4 1
+#define GL_VERSION_1_5 1
+#define GL_VERSION_2_0 1
+#define GL_VERSION_2_1 1
+#define GL_VERSION_3_0 1
+#define GL_VERSION_3_1 1
+#define GL_VERSION_3_2 1
+#define GL_VERSION_3_3 1
+#define GL_VERSION_4_0 1
+#define GL_VERSION_4_1 1
+#define GL_VERSION_4_2 1
+#define GL_VERSION_4_3 1
+#define GL_VERSION_4_4 1
+#define GL_VERSION_4_5 1
+#define GL_VERSION_4_6 1
+
 /* Data types */
 
 #include <KHR/khrplatform.h>
diff --git a/src/MagnumExternal/OpenGL/GL/flextGL.h.template b/src/MagnumExternal/OpenGL/GL/flextGL.h.template
index a9217af6c..471ff1a12 100644
--- a/src/MagnumExternal/OpenGL/GL/flextGL.h.template
+++ b/src/MagnumExternal/OpenGL/GL/flextGL.h.template
@@ -94,6 +94,15 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+@for cat,_ in functions:
+@if cat.startswith('VERSION_'):
+#define GL_@cat 1
+@endif
+@end
+
 /* Data types */
 
 @passthru
diff --git a/src/MagnumExternal/OpenGL/GLES2/Emscripten/flextGLEmscripten.h.template b/src/MagnumExternal/OpenGL/GLES2/Emscripten/flextGLEmscripten.h.template
index 0849a6079..ac895ef8b 100644
--- a/src/MagnumExternal/OpenGL/GLES2/Emscripten/flextGLEmscripten.h.template
+++ b/src/MagnumExternal/OpenGL/GLES2/Emscripten/flextGLEmscripten.h.template
@@ -56,6 +56,15 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+@for cat,_ in functions:
+@if cat.startswith('ES_VERSION_'):
+#define GL_@cat 1
+@endif
+@end
+
 /* Data types */
 
 @passthru
diff --git a/src/MagnumExternal/OpenGL/GLES2/flextGL.h b/src/MagnumExternal/OpenGL/GLES2/flextGL.h
index f8a72db4a..e4f36bada 100644
--- a/src/MagnumExternal/OpenGL/GLES2/flextGL.h
+++ b/src/MagnumExternal/OpenGL/GLES2/flextGL.h
@@ -80,6 +80,11 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+#define GL_ES_VERSION_2_0 1
+
 /* Data types */
 
 #include <KHR/khrplatform.h>
diff --git a/src/MagnumExternal/OpenGL/GLES2/flextGL.h.template b/src/MagnumExternal/OpenGL/GLES2/flextGL.h.template
index b465c2740..751c3a1d2 100644
--- a/src/MagnumExternal/OpenGL/GLES2/flextGL.h.template
+++ b/src/MagnumExternal/OpenGL/GLES2/flextGL.h.template
@@ -81,6 +81,15 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+@for cat,_ in functions:
+@if cat.startswith('ES_VERSION_'):
+#define GL_@cat 1
+@endif
+@end
+
 /* Data types */
 
 @passthru
diff --git a/src/MagnumExternal/OpenGL/GLES2/flextGLEmscripten.h b/src/MagnumExternal/OpenGL/GLES2/flextGLEmscripten.h
index eb47a02cd..6e42e6d2a 100644
--- a/src/MagnumExternal/OpenGL/GLES2/flextGLEmscripten.h
+++ b/src/MagnumExternal/OpenGL/GLES2/flextGLEmscripten.h
@@ -55,6 +55,11 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+#define GL_ES_VERSION_2_0 1
+
 /* Data types */
 
 #include <KHR/khrplatform.h>
diff --git a/src/MagnumExternal/OpenGL/GLES3/Emscripten/flextGLEmscripten.h.template b/src/MagnumExternal/OpenGL/GLES3/Emscripten/flextGLEmscripten.h.template
index fbf2a7d73..d0c9e79c4 100644
--- a/src/MagnumExternal/OpenGL/GLES3/Emscripten/flextGLEmscripten.h.template
+++ b/src/MagnumExternal/OpenGL/GLES3/Emscripten/flextGLEmscripten.h.template
@@ -61,6 +61,15 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+@for cat,_ in functions:
+@if cat.startswith('ES_VERSION_'):
+#define GL_@cat 1
+@endif
+@end
+
 /* Data types */
 
 @passthru
diff --git a/src/MagnumExternal/OpenGL/GLES3/flextGL.h b/src/MagnumExternal/OpenGL/GLES3/flextGL.h
index c83e6ba00..49e585fad 100644
--- a/src/MagnumExternal/OpenGL/GLES3/flextGL.h
+++ b/src/MagnumExternal/OpenGL/GLES3/flextGL.h
@@ -84,6 +84,14 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+#define GL_ES_VERSION_2_0 1
+#define GL_ES_VERSION_3_0 1
+#define GL_ES_VERSION_3_1 1
+#define GL_ES_VERSION_3_2 1
+
 /* Data types */
 
 #include <KHR/khrplatform.h>
diff --git a/src/MagnumExternal/OpenGL/GLES3/flextGL.h.template b/src/MagnumExternal/OpenGL/GLES3/flextGL.h.template
index 9b58f8fd0..a16222d75 100644
--- a/src/MagnumExternal/OpenGL/GLES3/flextGL.h.template
+++ b/src/MagnumExternal/OpenGL/GLES3/flextGL.h.template
@@ -85,6 +85,15 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+@for cat,_ in functions:
+@if cat.startswith('ES_VERSION_'):
+#define GL_@cat 1
+@endif
+@end
+
 /* Data types */
 
 @passthru
diff --git a/src/MagnumExternal/OpenGL/GLES3/flextGLEmscripten.h b/src/MagnumExternal/OpenGL/GLES3/flextGLEmscripten.h
index 77fc47a79..a1b157382 100644
--- a/src/MagnumExternal/OpenGL/GLES3/flextGLEmscripten.h
+++ b/src/MagnumExternal/OpenGL/GLES3/flextGLEmscripten.h
@@ -60,6 +60,12 @@ void flextGLInit(Magnum::GL::Context& context);
 #define GLAPI extern
 #endif
 
+/* GL version defines. Needs to be done to avoid conflicting definitions with
+   other GL headers (such as qopengl.h in Qt, which has different definition
+   for GLsizei and GLintptr on Windows) */
+#define GL_ES_VERSION_2_0 1
+#define GL_ES_VERSION_3_0 1
+
 /* Data types */
 
 #include <KHR/khrplatform.h>