diff --git a/packages/components/CHANGELOG.md b/packages/components/CHANGELOG.md
index 1e21a6f343b42a..51031d70ab6871 100644
--- a/packages/components/CHANGELOG.md
+++ b/packages/components/CHANGELOG.md
@@ -19,6 +19,7 @@
 
 ### Internal
 
+-   Add `getOverlayLegacySlot()` helper and matching `.wp-overlay-legacy` styles. Lazily mounts a body-level container at `z-index: 99997` with `isolation: isolate` to be used as the portal target for legacy overlays in a follow-up. No runtime behavior change yet.
 -   `NavigableContainer`: Refactor from class component to function component with hooks ([#77171](https://github.com/WordPress/gutenberg/pull/77171)).
 -   `Menu`: Refactor `Menu.Popover` to use Ariakit’s `render` prop, wrapping content in `MenuMotionRoot` (motion styles) and `MenuSurface` (panel layout and `variant` chrome) ([#77460](https://github.com/WordPress/gutenberg/pull/77460)).
 -   Fix types for TypeScript 7.0 ([#77177](https://github.com/WordPress/gutenberg/pull/77177)).
diff --git a/packages/components/src/style.scss b/packages/components/src/style.scss
index d316d1716f2e90..110652cccaa4bd 100644
--- a/packages/components/src/style.scss
+++ b/packages/components/src/style.scss
@@ -46,6 +46,7 @@
 @use "./toolbar/toolbar-button/style.scss" as *;
 @use "./toolbar/toolbar-group/style.scss" as *;
 @use "./tooltip/style.scss" as *;
+@use "./utils/overlay-legacy-slot/style.scss" as *;
 @use "./validated-form-controls/style.scss" as *;
 
 // Include the default WP Components color variables.
diff --git a/packages/components/src/utils/overlay-legacy-slot/index.ts b/packages/components/src/utils/overlay-legacy-slot/index.ts
new file mode 100644
index 00000000000000..85d924d9dcd883
--- /dev/null
+++ b/packages/components/src/utils/overlay-legacy-slot/index.ts
@@ -0,0 +1,67 @@
+/**
+ * Class name applied to the overlay legacy slot element.
+ */
+export const OVERLAY_LEGACY_SLOT_CLASSNAME = 'wp-overlay-legacy';
+
+/**
+ * Resolves the top-level document so that overlays portaling from inside an
+ * iframe still land in the parent document's slot. Falls back to the current
+ * document if `window.top` is inaccessible (cross-origin restrictions).
+ */
+function getTopLevelDocument(): Document {
+	try {
+		return window.top?.document ?? window.document;
+	} catch {
+		// Cross-origin access to `window.top.document` throws — use the
+		// current frame's document as a safe fallback.
+		return window.document;
+	}
+}
+
+let cachedSlot: HTMLDivElement | null = null;
+
+/**
+ * Lazily creates and returns the overlay legacy slot — a body-level container
+ * used as the portal target for legacy `@wordpress/components` overlays
+ * (Modal, Popover, Tooltip, Snackbar, Draggable clone).
+ *
+ * The slot has `z-index: 99997` and `isolation: isolate`, establishing a
+ * stacking context that sits below the WP admin bar (99,999) and below the
+ * overlay prime slot used by `@wordpress/ui` leaf overlays (99,998). Per-
+ * overlay z-indexes inside the slot continue to control relative ordering
+ * (Tooltip > Popover > Modal), but those values now stack relative to the
+ * slot rather than the document body.
+ *
+ * The slot is created in the top-level document (not the iframe's own
+ * document) so that overlays portaling from inside an iframe land in the
+ * parent document's slot. The element is cached and reused across calls; if
+ * the cached element has been detached or belongs to a different document,
+ * a fresh element is created.
+ *
+ * @return The overlay legacy slot element.
+ */
+export function getOverlayLegacySlot(): HTMLDivElement {
+	const doc = getTopLevelDocument();
+
+	if (
+		cachedSlot &&
+		cachedSlot.isConnected &&
+		cachedSlot.ownerDocument === doc
+	) {
+		return cachedSlot;
+	}
+
+	const existing = doc.body.querySelector< HTMLDivElement >(
+		`.${ OVERLAY_LEGACY_SLOT_CLASSNAME }`
+	);
+	if ( existing ) {
+		cachedSlot = existing;
+		return existing;
+	}
+
+	const slot = doc.createElement( 'div' );
+	slot.className = OVERLAY_LEGACY_SLOT_CLASSNAME;
+	doc.body.append( slot );
+	cachedSlot = slot;
+	return slot;
+}
diff --git a/packages/components/src/utils/overlay-legacy-slot/style.scss b/packages/components/src/utils/overlay-legacy-slot/style.scss
new file mode 100644
index 00000000000000..be1d679e9a78c1
--- /dev/null
+++ b/packages/components/src/utils/overlay-legacy-slot/style.scss
@@ -0,0 +1,21 @@
+// Overlay legacy slot — body-level container that establishes a stacking
+// context for legacy `@wordpress/components` overlays (Modal, Popover,
+// Tooltip, Snackbar, Draggable clone).
+//
+// Sits below the WP admin bar (99,999) and below the overlay prime slot
+// used by `@wordpress/ui` leaf overlays (99,998). Declared outside any CSS
+// layer for predictable plugin-override semantics.
+//
+// `position: fixed; top: 0; left: 0` (with no width/height) makes the slot
+// a zero-sized anchor at the viewport origin. Descendants positioned with
+// viewport-relative coordinates (via floating-ui or similar) resolve
+// correctly. `isolation: isolate` creates a stacking context regardless of
+// the slot's positioning. The slot has no size, so it never intercepts
+// pointer events on its own.
+.wp-overlay-legacy {
+	position: fixed;
+	top: 0;
+	left: 0;
+	z-index: 99997;
+	isolation: isolate;
+}
diff --git a/packages/components/src/utils/overlay-legacy-slot/test/index.ts b/packages/components/src/utils/overlay-legacy-slot/test/index.ts
new file mode 100644
index 00000000000000..12d10d42346a73
--- /dev/null
+++ b/packages/components/src/utils/overlay-legacy-slot/test/index.ts
@@ -0,0 +1,57 @@
+/**
+ * Internal dependencies
+ */
+import { getOverlayLegacySlot, OVERLAY_LEGACY_SLOT_CLASSNAME } from '..';
+
+describe( 'getOverlayLegacySlot', () => {
+	afterEach( () => {
+		document.body
+			.querySelectorAll( `.${ OVERLAY_LEGACY_SLOT_CLASSNAME }` )
+			.forEach( ( element ) => element.remove() );
+	} );
+
+	it( 'lazily creates a slot element on first call and appends it to the document body', () => {
+		expect(
+			document.body.querySelector( `.${ OVERLAY_LEGACY_SLOT_CLASSNAME }` )
+		).toBeNull();
+
+		const slot = getOverlayLegacySlot();
+
+		expect( slot ).toBeInstanceOf( HTMLDivElement );
+		expect( slot ).toHaveClass( OVERLAY_LEGACY_SLOT_CLASSNAME );
+		expect( slot.parentElement ).toBe( document.body );
+	} );
+
+	it( 'caches the singleton across calls', () => {
+		const first = getOverlayLegacySlot();
+		const second = getOverlayLegacySlot();
+
+		expect( second ).toBe( first );
+		expect(
+			document.body.querySelectorAll(
+				`.${ OVERLAY_LEGACY_SLOT_CLASSNAME }`
+			)
+		).toHaveLength( 1 );
+	} );
+
+	it( 'reuses an existing slot element already present in the DOM', () => {
+		const preexisting = document.createElement( 'div' );
+		preexisting.className = OVERLAY_LEGACY_SLOT_CLASSNAME;
+		document.body.append( preexisting );
+
+		const slot = getOverlayLegacySlot();
+
+		expect( slot ).toBe( preexisting );
+	} );
+
+	it( 'recreates the slot if the cached element has been detached', () => {
+		const first = getOverlayLegacySlot();
+		first.remove();
+
+		const second = getOverlayLegacySlot();
+
+		expect( second ).not.toBe( first );
+		expect( second.isConnected ).toBe( true );
+		expect( second.parentElement ).toBe( document.body );
+	} );
+} );