diff --git a/packages/components/CHANGELOG.md b/packages/components/CHANGELOG.md
index 803c01b8f4336c..f99fd10a1737c9 100644
--- a/packages/components/CHANGELOG.md
+++ b/packages/components/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Bug Fixes
+
+-   `SandBox`: Fix the viewport-unit (`vh`, `vw`, etc.) stripping so user-supplied HTML using these units in `width`/`height` no longer triggers a runaway resize loop in the preview ([#78677](https://github.com/WordPress/gutenberg/pull/78677)).
+
 ## 35.0.0 (2026-06-10)
 
 ### Breaking Changes
diff --git a/packages/components/src/sandbox/index.tsx b/packages/components/src/sandbox/index.tsx
index b5b2f55753098f..bd8290437208db 100644
--- a/packages/components/src/sandbox/index.tsx
+++ b/packages/components/src/sandbox/index.tsx
@@ -17,6 +17,23 @@ import type { SandBoxProps } from './types';
 
 type SandBoxContentProps = Omit< SandBoxProps, 'allowSameOrigin' >;
 
+/**
+ * Matches CSS viewport-relative length values such as `100vh`, `50.5vw`,
+ * and `.5dvh`. Used to strip viewport units from user-supplied HTML inside
+ * the sandbox iframe, because those units are relative to the iframe's
+ * own size and would create a measurement feedback loop with the
+ * resize observer.
+ *
+ * Exported for tests. NOTE: an identical regex literal is duplicated
+ * inside `observeAndResizeJS` below because that function is serialized
+ * via `.toString()` and embedded into the iframe's `srcdoc` — it has no
+ * access to this module's scope at runtime. If you change one, change
+ * the other; the "is embedded in the sandbox iframe srcdoc" test
+ * guards against drift.
+ */
+export const VIEWPORT_UNIT_VALUE_REGEX =
+	/^\d*\.?\d+(?:vw|vh|svw|lvw|dvw|svh|lvh|dvh|vi|svi|lvi|dvi|vb|svb|lvb|dvb|vmin|svmin|lvmin|dvmin|vmax|svmax|lvmax|dvmax)$/;
+
 const observeAndResizeJS = function () {
 	const { MutationObserver } = window;
 
@@ -58,7 +75,7 @@ const observeAndResizeJS = function () {
 				[ 'width', 'height', 'minHeight', 'maxHeight' ] as const
 			 ).forEach( function ( style ) {
 				if (
-					/^\\d+(vw|vh|svw|lvw|dvw|svh|lvh|dvh|vi|svi|lvi|dvi|vb|svb|lvb|dvb|vmin|svmin|lvmin|dvmin|vmax|svmax|lvmax|dvmax)$/.test(
+					/^\d*\.?\d+(?:vw|vh|svw|lvw|dvw|svh|lvh|dvh|vi|svi|lvi|dvi|vb|svb|lvb|dvb|vmin|svmin|lvmin|dvmin|vmax|svmax|lvmax|dvmax)$/.test(
 						ruleOrNode.style[ style ]
 					)
 				) {
diff --git a/packages/components/src/sandbox/test/index.tsx b/packages/components/src/sandbox/test/index.tsx
index 70af01fd094f2a..e970ef564c0545 100644
--- a/packages/components/src/sandbox/test/index.tsx
+++ b/packages/components/src/sandbox/test/index.tsx
@@ -11,7 +11,7 @@ import { useState } from '@wordpress/element';
 /**
  * Internal dependencies
  */
-import SandBox from '..';
+import SandBox, { VIEWPORT_UNIT_VALUE_REGEX } from '..';
 
 describe( 'SandBox', () => {
 	const TestWrapper = () => {
@@ -109,4 +109,47 @@ describe( 'SandBox', () => {
 			expect.stringContaining( 'https://another.super.embed' )
 		);
 	} );
+
+	describe( 'VIEWPORT_UNIT_VALUE_REGEX', () => {
+		it.each( [
+			'100vh',
+			'50vw',
+			'0vh',
+			'50.5vh',
+			'.5vh',
+			'100dvh',
+			'50svw',
+			'1lvi',
+			'100vmin',
+			'100vmax',
+		] )( 'matches viewport unit value %s', ( value ) => {
+			expect( VIEWPORT_UNIT_VALUE_REGEX.test( value ) ).toBe( true );
+		} );
+
+		it.each( [
+			'100px',
+			'50%',
+			'100',
+			'vh',
+			'.vh',
+			'calc(100vh - 10px)',
+			'100 vh',
+			'',
+		] )( 'does not match %s', ( value ) => {
+			expect( VIEWPORT_UNIT_VALUE_REGEX.test( value ) ).toBe( false );
+		} );
+
+		it( 'is embedded in the sandbox iframe srcdoc', () => {
+			// Guards against drift between the exported constant and
+			// the copy inlined into `observeAndResizeJS`, which is
+			// serialized via `.toString()` into the iframe srcdoc and
+			// cannot reference module-scope values at runtime.
+			render( <SandBox html="<p>x</p>" title="Regex Sync Test" /> );
+			const iframe =
+				screen.getByTitle< HTMLIFrameElement >( 'Regex Sync Test' );
+			const srcDoc = iframe.getAttribute( 'srcdoc' ) ?? '';
+
+			expect( srcDoc ).toContain( VIEWPORT_UNIT_VALUE_REGEX.source );
+		} );
+	} );
 } );