diff --git a/src/components/overlay_mask/overlay_mask.tsx b/src/components/overlay_mask/overlay_mask.tsx
index 2f6fe007ca7..8bdbab666ed 100644
--- a/src/components/overlay_mask/overlay_mask.tsx
+++ b/src/components/overlay_mask/overlay_mask.tsx
@@ -91,7 +91,8 @@ export const EuiOverlayMask: FunctionComponent<EuiOverlayMaskProps> = ({
   useEffect(() => {
     if (!overlayMaskNode) return;
     overlayMaskNode.className = classNames('euiOverlayMask', className);
-  }, [overlayMaskNode, className]);
+    overlayMaskNode.dataset.relativeToHeader = headerZindexLocation;
+  }, [overlayMaskNode, className, headerZindexLocation]);
 
   return (
     <EuiPortal portalRef={combinedMaskRef}>
diff --git a/upcoming_changelogs/6289.md b/upcoming_changelogs/6289.md
new file mode 100644
index 00000000000..5a38a09b6be
--- /dev/null
+++ b/upcoming_changelogs/6289.md
@@ -0,0 +1,3 @@
+**Bug fixes**
+
+- Fixed `EuiOverlayMask` to set a `[data-relative-to-header=above|below]` attribute to replace the `--aboveHeader` and `--belowHeader` classNames removed in its Emotion conversion