diff --git a/include/GLFW/glfw3.h b/include/GLFW/glfw3.h
index 79b06288..5fbaf1b4 100644
--- a/include/GLFW/glfw3.h
+++ b/include/GLFW/glfw3.h
@@ -1102,6 +1102,7 @@ extern "C" {
  *  [window hint](@ref GLFW_SCALE_FRAMEBUFFER_hint).
  */
 #define GLFW_SCALE_FRAMEBUFFER      0x0002200D
+#define GLFW_FULLSCREEN             0x0002200E
 /*! @brief Legacy name for compatibility.
  *
  *  This is an alias for the
diff --git a/src/internal.h b/src/internal.h
index 4f097aa8..320e6724 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -747,6 +747,7 @@ struct _GLFWplatform
     void (*setWindowFloating)(_GLFWwindow*,GLFWbool);
     void (*setWindowOpacity)(_GLFWwindow*,float);
     void (*setWindowMousePassthrough)(_GLFWwindow*,GLFWbool);
+    void (*getWindowIsFullscreen) (_GLFWwindow*);
     void (*pollEvents)(void);
     void (*waitEvents)(void);
     void (*waitEventsTimeout)(double);
diff --git a/src/window.c b/src/window.c
index e03121a4..140e11b2 100644
--- a/src/window.c
+++ b/src/window.c
@@ -173,6 +173,10 @@ void _glfwInputWindowMonitor(_GLFWwindow* window, _GLFWmonitor* monitor)
     window->monitor = monitor;
 }
 
+GLFWbool _glfwWindowGetIsFullscreen(_GLFWwindow* window) {
+    return _glfwGetIsWindowFullscreenX11(window);
+}
+
 //////////////////////////////////////////////////////////////////////////
 //////                        GLFW public API                       //////
 //////////////////////////////////////////////////////////////////////////
@@ -900,6 +904,8 @@ GLFWAPI int glfwGetWindowAttrib(GLFWwindow* handle, int attrib)
             return window->focusOnShow;
         case GLFW_MOUSE_PASSTHROUGH:
             return window->mousePassthrough;
+        case GLFW_FULLSCREEN:
+            return _glfwWindowGetIsFullscreen(window);
         case GLFW_TRANSPARENT_FRAMEBUFFER:
             return _glfw.platform.framebufferTransparent(window);
         case GLFW_RESIZABLE:
diff --git a/src/x11_platform.h b/src/x11_platform.h
index 30326c5b..ac17671f 100644
--- a/src/x11_platform.h
+++ b/src/x11_platform.h
@@ -934,6 +934,7 @@ void _glfwSetWindowFloatingX11(_GLFWwindow* window, GLFWbool enabled);
 float _glfwGetWindowOpacityX11(_GLFWwindow* window);
 void _glfwSetWindowOpacityX11(_GLFWwindow* window, float opacity);
 void _glfwSetWindowMousePassthroughX11(_GLFWwindow* window, GLFWbool enabled);
+GLFWbool _glfwGetIsWindowFullscreenX11(_GLFWwindow* window);
 
 void _glfwSetRawMouseMotionX11(_GLFWwindow *window, GLFWbool enabled);
 GLFWbool _glfwRawMouseMotionSupportedX11(void);
diff --git a/src/x11_window.c b/src/x11_window.c
index 322349f0..06324faa 100644
--- a/src/x11_window.c
+++ b/src/x11_window.c
@@ -26,6 +26,7 @@
 //========================================================================
 
 #include "internal.h"
+#include <X11/Xlib.h>
 
 #if defined(_GLFW_X11)
 
@@ -2729,6 +2730,27 @@ void _glfwSetWindowMousePassthroughX11(_GLFWwindow* window, GLFWbool enabled)
     }
 }
 
+GLFWbool _glfwGetIsWindowFullscreenX11(_GLFWwindow* window)
+{
+    Atom atom = XInternAtom(_glfw.x11.display, "_NET_WM_STATE_FULLSCREEN", 0);
+
+    unsigned char prop[32] = {};
+    XGetWindowProperty(
+        _glfw.x11.display,
+        window->x11.handle,
+        atom, 
+        0,
+        1, // 32 bits
+        0,
+        NULL,
+        NULL,
+        NULL,
+        NULL,
+        NULL,
+        (unsigned char**)&prop
+    );
+}
+
 float _glfwGetWindowOpacityX11(_GLFWwindow* window)
 {
     float opacity = 1.f;
diff --git a/tests/window_fullscreen_x11.c b/tests/window_fullscreen_x11.c
new file mode 100644
index 00000000..8b10036e
--- /dev/null
+++ b/tests/window_fullscreen_x11.c
@@ -0,0 +1,6 @@
+#include <GLFW/glfw3.h>
+#include <stdio.h>
+
+int main() {
+    glfwGetWindowAttrib();
+}