diff --git a/src/gl/renderer/gl_renderer.cpp b/src/gl/renderer/gl_renderer.cpp
index 0a21f1e01d..57d17dfb48 100644
--- a/src/gl/renderer/gl_renderer.cpp
+++ b/src/gl/renderer/gl_renderer.cpp
@@ -286,7 +286,7 @@ void FGLRenderer::SetOutputViewport(GL_IRECT *bounds)
 	mSceneViewport.height = height;
 
 	// Scale viewports to fit letterbox
-	if (gl_scale_viewport || !FGLRenderBuffers::IsEnabled())
+	if ((gl_scale_viewport && !framebuffer->IsFullscreen()) || !FGLRenderBuffers::IsEnabled())
 	{
 		mScreenViewport.width = mOutputLetterbox.width;
 		mScreenViewport.height = mOutputLetterbox.height;