diff --git a/winit-appkit/src/lib.rs b/winit-appkit/src/lib.rs
index 8548f8f73..3ff9c82f4 100644
--- a/winit-appkit/src/lib.rs
+++ b/winit-appkit/src/lib.rs
@@ -188,6 +188,25 @@ pub trait WindowExtMacOS {
 
     /// Getter for the [`WindowExtMacOS::set_unified_titlebar`].
     fn unified_titlebar(&self) -> bool;
+
+    /// Sets whether the window can be shown on the same Space as a fullscreen window.
+    ///
+    /// This corresponds to [`NSWindowCollectionBehaviorFullScreenAuxiliary`], and is useful
+    /// for floating palettes, inspectors and other secondary windows accompanying a fullscreen
+    /// window. Without it, ordering a new window on screen while another window of the
+    /// application is fullscreen on the active Space makes macOS switch Spaces or attempt
+    /// Split View tiling.
+    ///
+    /// A window marked as fullscreen auxiliary cannot itself enter (native) fullscreen;
+    /// [`Window::set_fullscreen`] will warn and do nothing. Call
+    /// `set_fullscreen_auxiliary(false)` first if you want to make the window fullscreen.
+    ///
+    /// [`NSWindowCollectionBehaviorFullScreenAuxiliary`]: https://developer.apple.com/documentation/appkit/nswindow/collectionbehavior-swift.struct/fullscreenauxiliary?language=objc
+    /// [`Window::set_fullscreen`]: winit_core::window::Window::set_fullscreen
+    fn set_fullscreen_auxiliary(&self, fullscreen_auxiliary: bool);
+
+    /// Getter for the [`WindowExtMacOS::set_fullscreen_auxiliary`].
+    fn fullscreen_auxiliary(&self) -> bool;
 }
 
 impl WindowExtMacOS for dyn Window + '_ {
@@ -298,6 +317,18 @@ impl WindowExtMacOS for dyn Window + '_ {
         let window = self.cast_ref::<AppKitWindow>().unwrap();
         window.maybe_wait_on_main(|w| w.unified_titlebar())
     }
+
+    #[inline]
+    fn set_fullscreen_auxiliary(&self, fullscreen_auxiliary: bool) {
+        let window = self.cast_ref::<AppKitWindow>().unwrap();
+        window.maybe_wait_on_main(move |w| w.set_fullscreen_auxiliary(fullscreen_auxiliary))
+    }
+
+    #[inline]
+    fn fullscreen_auxiliary(&self) -> bool {
+        let window = self.cast_ref::<AppKitWindow>().unwrap();
+        window.maybe_wait_on_main(|w| w.fullscreen_auxiliary())
+    }
 }
 
 /// Corresponds to `NSApplicationActivationPolicy`.
@@ -342,6 +373,7 @@ pub struct WindowAttributesMacOS {
     pub(crate) borderless_game: bool,
     pub(crate) unified_titlebar: bool,
     pub(crate) panel: bool,
+    pub(crate) fullscreen_auxiliary: bool,
 }
 
 impl WindowAttributesMacOS {
@@ -449,6 +481,17 @@ impl WindowAttributesMacOS {
         self.panel = panel;
         self
     }
+
+    /// See [`WindowExtMacOS::set_fullscreen_auxiliary`] for details on what this means if set.
+    ///
+    /// Contrary to the runtime setter, setting this attribute guarantees that the collection
+    /// behavior is already in place when the window is first ordered on screen, which is
+    /// required to avoid disturbing an active fullscreen Space.
+    #[inline]
+    pub fn with_fullscreen_auxiliary(mut self, fullscreen_auxiliary: bool) -> Self {
+        self.fullscreen_auxiliary = fullscreen_auxiliary;
+        self
+    }
 }
 
 impl Default for WindowAttributesMacOS {
@@ -469,6 +512,7 @@ impl Default for WindowAttributesMacOS {
             borderless_game: false,
             unified_titlebar: false,
             panel: false,
+            fullscreen_auxiliary: false,
         }
     }
 }
diff --git a/winit-appkit/src/window_delegate.rs b/winit-appkit/src/window_delegate.rs
index 9a6515016..8bc14e3df 100644
--- a/winit-appkit/src/window_delegate.rs
+++ b/winit-appkit/src/window_delegate.rs
@@ -24,9 +24,9 @@ use objc2_app_kit::{
     NSDraggingSession, NSDraggingSource, NSPasteboardTypeFileURL, NSPasteboardTypeHTML,
     NSPasteboardTypePNG, NSPasteboardTypeSound, NSPasteboardTypeString, NSPasteboardTypeTIFF,
     NSRequestUserAttentionType, NSScreen, NSToolbar, NSView, NSViewFrameDidChangeNotification,
-    NSWindow, NSWindowButton, NSWindowDelegate, NSWindowLevel, NSWindowOcclusionState,
-    NSWindowOrderingMode, NSWindowSharingType, NSWindowStyleMask, NSWindowTabbingMode,
-    NSWindowTitleVisibility, NSWindowToolbarStyle,
+    NSWindow, NSWindowButton, NSWindowCollectionBehavior, NSWindowDelegate, NSWindowLevel,
+    NSWindowOcclusionState, NSWindowOrderingMode, NSWindowSharingType, NSWindowStyleMask,
+    NSWindowTabbingMode, NSWindowTitleVisibility, NSWindowToolbarStyle,
 };
 use objc2_core_foundation::{CGFloat, CGPoint};
 use objc2_core_graphics::{
@@ -829,6 +829,14 @@ fn new_window(
         if !macos_attrs.has_shadow {
             window.setHasShadow(false);
         }
+        if macos_attrs.fullscreen_auxiliary {
+            // This must happen before the window is ordered on screen (below, and in
+            // `WindowDelegate::new`), so that showing the window doesn't make macOS
+            // switch away from an active fullscreen Space or attempt Split View tiling.
+            window.setCollectionBehavior(
+                window.collectionBehavior() | NSWindowCollectionBehavior::FullScreenAuxiliary,
+            );
+        }
         // Popups are positioned relative to their parent in `WindowDelegate::new`.
         if attrs.position.is_none() && !is_popup {
             window.center();
@@ -1646,6 +1654,20 @@ impl WindowDelegate {
         if self.ivars().is_simple_fullscreen.get() {
             return;
         }
+        if fullscreen.is_some()
+            && self
+                .window()
+                .collectionBehavior()
+                .contains(NSWindowCollectionBehavior::FullScreenAuxiliary)
+        {
+            // `toggleFullScreen:` is silently ignored on such windows, which would leave our
+            // internal fullscreen state out of sync, so bail out early instead.
+            warn!(
+                "cannot fullscreen a window marked as fullscreen auxiliary; call \
+                 `set_fullscreen_auxiliary(false)` first"
+            );
+            return;
+        }
         if self.ivars().in_fullscreen_transition.get() {
             // We can't set fullscreen here.
             // Set fullscreen after transition.
@@ -2245,6 +2267,24 @@ impl WindowExtMacOS for WindowDelegate {
 
         window.toolbar().is_some() && window.toolbarStyle() == NSWindowToolbarStyle::Unified
     }
+
+    #[inline]
+    fn set_fullscreen_auxiliary(&self, fullscreen_auxiliary: bool) {
+        let window = self.window();
+        let behavior = window.collectionBehavior();
+        if fullscreen_auxiliary {
+            window
+                .setCollectionBehavior(behavior | NSWindowCollectionBehavior::FullScreenAuxiliary);
+        } else {
+            window
+                .setCollectionBehavior(behavior - NSWindowCollectionBehavior::FullScreenAuxiliary);
+        }
+    }
+
+    #[inline]
+    fn fullscreen_auxiliary(&self) -> bool {
+        self.window().collectionBehavior().contains(NSWindowCollectionBehavior::FullScreenAuxiliary)
+    }
 }
 
 const DEFAULT_STANDARD_FRAME: NSRect =
diff --git a/winit/src/changelog/unreleased.md b/winit/src/changelog/unreleased.md
index 420859434..e2f7326ea 100644
--- a/winit/src/changelog/unreleased.md
+++ b/winit/src/changelog/unreleased.md
@@ -52,6 +52,11 @@ changelog entry.
 - On Wayland, added `HoldGesture` event for multi-finger hold gestures
 - On Wayland, added ext-background-effect-v1 support.
 - On Wayland, Windows and macOS, added native popups (`WindowType::Popup`).
+- On macOS, add `WindowAttributesMacOS::with_fullscreen_auxiliary` and
+  `WindowExtMacOS::set_fullscreen_auxiliary` / `WindowExtMacOS::fullscreen_auxiliary`, allowing a
+  window to be shown on the same Space as a fullscreen window
+  (`NSWindowCollectionBehaviorFullScreenAuxiliary`) instead of triggering a Space switch or Split
+  View tiling.
 
 ### Changed