diff --git a/.changeset/redirect-catch-all-hardening.md b/.changeset/redirect-catch-all-hardening.md
new file mode 100644
index 000000000000..4566f24c0d61
--- /dev/null
+++ b/.changeset/redirect-catch-all-hardening.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Hardens config-based redirects with catch-all parameters to prevent producing protocol-relative URLs (e.g. `//example.com`) in the `Location` header
diff --git a/packages/astro/src/core/redirects/render.ts b/packages/astro/src/core/redirects/render.ts
index eba4395b36ca..00a4543c109b 100644
--- a/packages/astro/src/core/redirects/render.ts
+++ b/packages/astro/src/core/redirects/render.ts
@@ -2,13 +2,15 @@ import type { RedirectConfig } from '../../types/public/index.js';
 import type { RenderContext } from '../render-context.js';
 import { getRouteGenerator } from '../routing/generator.js';
 
+function isExternalURL(url: string): boolean {
+	return url.startsWith('http://') || url.startsWith('https://') || url.startsWith('//');
+}
+
 export function redirectIsExternal(redirect: RedirectConfig): boolean {
 	if (typeof redirect === 'string') {
-		return redirect.startsWith('http://') || redirect.startsWith('https://');
+		return isExternalURL(redirect);
 	} else {
-		return (
-			redirect.destination.startsWith('http://') || redirect.destination.startsWith('https://')
-		);
+		return isExternalURL(redirect.destination);
 	}
 }
 
diff --git a/packages/astro/src/core/routing/generator.ts b/packages/astro/src/core/routing/generator.ts
index 94dcf4324465..7ee9d8806aac 100644
--- a/packages/astro/src/core/routing/generator.ts
+++ b/packages/astro/src/core/routing/generator.ts
@@ -1,3 +1,4 @@
+import { collapseDuplicateLeadingSlashes } from '@astrojs/internal-helpers/path';
 import type { AstroConfig } from '../../types/public/config.js';
 import type { RoutePart } from '../../types/public/internal.js';
 
@@ -41,7 +42,7 @@ function getParameter(part: RoutePart, params: Record<string, string | number>):
 function getSegment(segment: RoutePart[], params: Record<string, string | number>): string {
 	const segmentPath = segment.map((part) => getParameter(part, params)).join('');
 
-	return segmentPath ? '/' + segmentPath : '';
+	return segmentPath ? collapseDuplicateLeadingSlashes('/' + segmentPath) : '';
 }
 
 type RouteGenerator = (data?: any) => string;
diff --git a/packages/astro/test/units/redirects/open-redirect.test.js b/packages/astro/test/units/redirects/open-redirect.test.js
new file mode 100644
index 000000000000..61f27b612014
--- /dev/null
+++ b/packages/astro/test/units/redirects/open-redirect.test.js
@@ -0,0 +1,102 @@
+// @ts-check
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import { redirectIsExternal } from '../../../dist/core/redirects/render.js';
+import { getRouteGenerator } from '../../../dist/core/routing/generator.js';
+
+describe('Protocol-relative URLs in redirects', () => {
+	describe('redirectIsExternal', () => {
+		it('detects http:// as external', () => {
+			assert.equal(redirectIsExternal('http://evil.com'), true);
+		});
+
+		it('detects https:// as external', () => {
+			assert.equal(redirectIsExternal('https://evil.com'), true);
+		});
+
+		it('detects protocol-relative //evil.com as external', () => {
+			assert.equal(redirectIsExternal('//evil.com'), true);
+		});
+
+		it('detects protocol-relative //evil.com/path as external', () => {
+			assert.equal(redirectIsExternal('//evil.com/path'), true);
+		});
+
+		it('does not flag normal paths as external', () => {
+			assert.equal(redirectIsExternal('/about'), false);
+		});
+
+		it('does not flag root path as external', () => {
+			assert.equal(redirectIsExternal('/'), false);
+		});
+
+		it('detects protocol-relative URL in object form', () => {
+			assert.equal(redirectIsExternal({ destination: '//evil.com', status: 301 }), true);
+		});
+	});
+
+	describe('getRouteGenerator with root-level catch-all', () => {
+		it('does not produce protocol-relative URL when catch-all param contains leading slash', () => {
+			// Simulates destination '/[...slug]' — a single root-level catch-all segment
+			const segments = [[{ spread: true, content: '...slug', dynamic: true }]];
+			const generator = getRouteGenerator(segments, 'never');
+
+			// When the request is '/old//evil.com/', the catch-all captures '/evil.com'
+			const result = generator({ slug: '/evil.com' });
+
+			// The result must NOT be '//evil.com' (protocol-relative URL)
+			assert.ok(
+				!result.startsWith('//'),
+				`Expected result to not start with '//', got '${result}'`,
+			);
+		});
+
+		it('does not produce protocol-relative URL with trailing slash config', () => {
+			const segments = [[{ spread: true, content: '...slug', dynamic: true }]];
+			const generator = getRouteGenerator(segments, 'always');
+
+			const result = generator({ slug: '/evil.com' });
+
+			assert.ok(
+				!result.startsWith('//'),
+				`Expected result to not start with '//', got '${result}'`,
+			);
+		});
+
+		it('does not produce protocol-relative URL with subpath', () => {
+			const segments = [[{ spread: true, content: '...slug', dynamic: true }]];
+			const generator = getRouteGenerator(segments, 'never');
+
+			const result = generator({ slug: '/evil.com/phish' });
+
+			assert.ok(
+				!result.startsWith('//'),
+				`Expected result to not start with '//', got '${result}'`,
+			);
+		});
+
+		it('still produces correct paths for normal params', () => {
+			const segments = [[{ spread: true, content: '...slug', dynamic: true }]];
+			const generator = getRouteGenerator(segments, 'never');
+
+			assert.equal(generator({ slug: 'about' }), '/about');
+			assert.equal(generator({ slug: 'docs/getting-started' }), '/docs/getting-started');
+		});
+
+		it('non-root catch-all is not affected', () => {
+			// Simulates destination '/new/[...slug]' — catch-all under a prefix
+			const segments = [
+				[{ spread: false, content: 'new', dynamic: false }],
+				[{ spread: true, content: '...slug', dynamic: true }],
+			];
+			const generator = getRouteGenerator(segments, 'never');
+
+			// Even with a leading-slash param, the prefix prevents protocol-relative
+			const result = generator({ slug: '/evil.com' });
+			assert.ok(
+				!result.startsWith('//'),
+				`Expected result to not start with '//', got '${result}'`,
+			);
+		});
+	});
+});