diff --git a/.changeset/great-bags-heal.md b/.changeset/great-bags-heal.md
new file mode 100644
index 000000000000..eaf1dc6d55b4
--- /dev/null
+++ b/.changeset/great-bags-heal.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+feat: infer route parameter type from matcher's guard check if applicable
diff --git a/packages/kit/src/core/sync/write_types/index.js b/packages/kit/src/core/sync/write_types/index.js
index e5f9afb76c57..d21030e01109 100644
--- a/packages/kit/src/core/sync/write_types/index.js
+++ b/packages/kit/src/core/sync/write_types/index.js
@@ -188,10 +188,18 @@ function update_types(config, routes, route, to_delete = new Set()) {
 	// add 'Expand' helper
 	// Makes sure a type is "repackaged" and therefore more readable
 	declarations.push('type Expand<T> = T extends infer O ? { [K in keyof O]: O[K] } : never;');
+
+	// returns the predicate of a matcher's type guard - or string if there is no type guard
 	declarations.push(
-		`type RouteParams = { ${route.params
-			.map((param) => `${param.name}${param.optional ? '?' : ''}: string`)
-			.join('; ')} }`
+		// TS complains on infer U, which seems weird, therefore ts-ignore it
+		[
+			'// @ts-ignore',
+			'type MatcherParam<M> = M extends (param : string) => param is infer U ? U extends string ? U : string : string;'
+		].join('\n')
+	);
+
+	declarations.push(
+		'type RouteParams = ' + generate_params_type(route.params, outdir, config) + ';'
 	);
 
 	if (route.params.length > 0) {
@@ -265,7 +273,8 @@ function update_types(config, routes, route, to_delete = new Set()) {
 
 	if (route.layout) {
 		let all_pages_have_load = true;
-		const layout_params = new Set();
+		/** @type {import('types').RouteParam[]} */
+		const layout_params = [];
 		const ids = ['RouteId'];
 
 		route.layout.child_pages?.forEach((page) => {
@@ -274,7 +283,9 @@ function update_types(config, routes, route, to_delete = new Set()) {
 				if (leaf.route.page) ids.push(`"${leaf.route.id}"`);
 
 				for (const param of leaf.route.params) {
-					layout_params.add(param.name);
+					// skip if already added
+					if (layout_params.some((p) => p.name === param.name)) continue;
+					layout_params.push({ ...param, optional: true });
 				}
 
 				ensureProxies(page, leaf.proxies);
@@ -301,9 +312,7 @@ function update_types(config, routes, route, to_delete = new Set()) {
 		declarations.push(`type LayoutRouteId = ${ids.join(' | ')}`);
 
 		declarations.push(
-			`type LayoutParams = RouteParams & { ${Array.from(layout_params).map(
-				(param) => `${param}?: string`
-			)} }`
+			'type LayoutParams = RouteParams & ' + generate_params_type(layout_params, outdir, config)
 		);
 
 		const {
@@ -567,6 +576,28 @@ function replace_ext_with_js(file_path) {
 	return file_path.slice(0, -ext.length) + '.js';
 }
 
+/**
+ * @param {import('types').RouteParam[]} params
+ * @param {string} outdir
+ * @param {import('types').ValidatedConfig} config
+ */
+function generate_params_type(params, outdir, config) {
+	/** @param {string} matcher */
+	const path_to_matcher = (matcher) =>
+		posixify(path.relative(outdir, path.join(config.kit.files.params, matcher)));
+
+	return `{ ${params
+		.map(
+			(param) =>
+				`${param.name}${param.optional ? '?' : ''}: ${
+					param.matcher
+						? `MatcherParam<typeof import('${path_to_matcher(param.matcher)}').match>`
+						: 'string'
+				}`
+		)
+		.join('; ')} }`;
+}
+
 /**
  * @param {string} content
  * @param {boolean} is_server
diff --git a/packages/kit/src/core/sync/write_types/index.spec.js b/packages/kit/src/core/sync/write_types/index.spec.js
index 0c13aa04e863..d50a4d12fa82 100644
--- a/packages/kit/src/core/sync/write_types/index.spec.js
+++ b/packages/kit/src/core/sync/write_types/index.spec.js
@@ -18,7 +18,7 @@ async function run_test(dir) {
 	const initial = options({}, 'config');
 
 	initial.kit.files.assets = path.resolve(cwd, 'static');
-	initial.kit.files.params = path.resolve(cwd, 'params');
+	initial.kit.files.params = path.resolve(cwd, dir, 'params');
 	initial.kit.files.routes = path.resolve(cwd, dir);
 	initial.kit.outDir = path.resolve(cwd, path.join(dir, '.svelte-kit'));
 
@@ -40,6 +40,7 @@ test('Creates correct $types', async () => {
 	await run_test('layout-advanced');
 	await run_test('slugs');
 	await run_test('slugs-layout-not-all-pages-have-load');
+	await run_test('param-type-inference');
 	try {
 		execSync('pnpm testtypes', { cwd });
 	} catch (e) {
diff --git a/packages/kit/src/core/sync/write_types/test/param-type-inference/optional/[[optionalNarrowedParam=narrowed]]/+page.js b/packages/kit/src/core/sync/write_types/test/param-type-inference/optional/[[optionalNarrowedParam=narrowed]]/+page.js
new file mode 100644
index 000000000000..024289bef97e
--- /dev/null
+++ b/packages/kit/src/core/sync/write_types/test/param-type-inference/optional/[[optionalNarrowedParam=narrowed]]/+page.js
@@ -0,0 +1,11 @@
+/* eslint-disable */
+
+/** @type {import('../../.svelte-kit/types/src/core/sync/write_types/test/param-type-inference/optional/[[optionalNarrowedParam=narrowed]]/$types').PageLoad} */
+export function load({ params }) {
+	if (params.optionalNarrowedParam) {
+		/** @type {"a" | "b"} */
+		let a;
+		a = params.optionalNarrowedParam;
+		return { a };
+	}
+}
diff --git a/packages/kit/src/core/sync/write_types/test/param-type-inference/params/narrowed.js b/packages/kit/src/core/sync/write_types/test/param-type-inference/params/narrowed.js
new file mode 100644
index 000000000000..e8d48d1ca344
--- /dev/null
+++ b/packages/kit/src/core/sync/write_types/test/param-type-inference/params/narrowed.js
@@ -0,0 +1,5 @@
+/**
+ * @param {string} param
+ * @returns {param is "a" | "b"}
+ */
+export const match = (param) => ['a', 'b'].includes(param);
diff --git a/packages/kit/src/core/sync/write_types/test/param-type-inference/params/not_narrowed.js b/packages/kit/src/core/sync/write_types/test/param-type-inference/params/not_narrowed.js
new file mode 100644
index 000000000000..0e2f951afadd
--- /dev/null
+++ b/packages/kit/src/core/sync/write_types/test/param-type-inference/params/not_narrowed.js
@@ -0,0 +1,7 @@
+/* eslint-disable */
+
+/**
+ * @param {string} param
+ * @returns {boolean}
+ */
+export const match = (param) => true;
diff --git a/packages/kit/src/core/sync/write_types/test/param-type-inference/required/+layout.js b/packages/kit/src/core/sync/write_types/test/param-type-inference/required/+layout.js
new file mode 100644
index 000000000000..5a67f890cd8c
--- /dev/null
+++ b/packages/kit/src/core/sync/write_types/test/param-type-inference/required/+layout.js
@@ -0,0 +1,20 @@
+/* eslint-disable */
+
+/** @type {import('../.svelte-kit/types/src/core/sync/write_types/test/param-type-inference/required/$types').LayoutLoad} */
+export function load({ params }) {
+	if (params.narrowedParam) {
+		/** @type {"a" | "b"} */
+		const a = params.narrowedParam;
+	}
+
+	if (params.regularParam) {
+		/** @type {"a" | "b"} */
+		let a;
+
+		//@ts-expect-error
+		a = params.regularParam;
+
+		/** @type {string} b*/
+		const b = params.regularParam;
+	}
+}
diff --git a/packages/kit/src/core/sync/write_types/test/param-type-inference/required/[narrowedParam=narrowed]/+page.js b/packages/kit/src/core/sync/write_types/test/param-type-inference/required/[narrowedParam=narrowed]/+page.js
new file mode 100644
index 000000000000..395ae659aa1d
--- /dev/null
+++ b/packages/kit/src/core/sync/write_types/test/param-type-inference/required/[narrowedParam=narrowed]/+page.js
@@ -0,0 +1,8 @@
+/* eslint-disable */
+
+/** @type {import('../../.svelte-kit/types/src/core/sync/write_types/test/param-type-inference/required/[narrowedParam=narrowed]/$types').PageLoad} */
+export function load({ params }) {
+	/** @type {"a" | "b"} */
+	let a;
+	a = params.narrowedParam;
+}
diff --git a/packages/kit/src/core/sync/write_types/test/param-type-inference/required/[regularParam=not_narrowed]/+page.js b/packages/kit/src/core/sync/write_types/test/param-type-inference/required/[regularParam=not_narrowed]/+page.js
new file mode 100644
index 000000000000..6fd50653a023
--- /dev/null
+++ b/packages/kit/src/core/sync/write_types/test/param-type-inference/required/[regularParam=not_narrowed]/+page.js
@@ -0,0 +1,13 @@
+/* eslint-disable */
+
+/** @type {import('../../.svelte-kit/types/src/core/sync/write_types/test/param-type-inference/required/[regularParam=not_narrowed]/$types').PageLoad} */
+export function load({ params }) {
+	/** @type {string} a*/
+	const a = params.regularParam;
+
+	/** @type {"a" | "b"} b*/
+	let b;
+
+	//@ts-expect-error
+	b = params.regularParam;
+}
diff --git a/packages/kit/src/core/sync/write_types/test/param-type-inference/spread/[...spread=narrowed]/+page.js b/packages/kit/src/core/sync/write_types/test/param-type-inference/spread/[...spread=narrowed]/+page.js
new file mode 100644
index 000000000000..f26d10e3aa74
--- /dev/null
+++ b/packages/kit/src/core/sync/write_types/test/param-type-inference/spread/[...spread=narrowed]/+page.js
@@ -0,0 +1,8 @@
+/* eslint-disable */
+
+/** @type {import('../../.svelte-kit/types/src/core/sync/write_types/test/param-type-inference/spread/[...spread=narrowed]/$types').PageLoad} */
+export function load({ params }) {
+	/** @type {"a" | "b"} */
+	let a;
+	a = params.spread;
+}