diff --git a/.changeset/fix-svelte-map-undefined.md b/.changeset/fix-svelte-map-undefined.md
new file mode 100644
index 0000000000..fa981c46f8
--- /dev/null
+++ b/.changeset/fix-svelte-map-undefined.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: `SvelteMap` incorrectly handles keys with `undefined` values
diff --git a/packages/svelte/src/reactivity/map.js b/packages/svelte/src/reactivity/map.js
index 014b5e7c7c..48d06a05a7 100644
--- a/packages/svelte/src/reactivity/map.js
+++ b/packages/svelte/src/reactivity/map.js
@@ -98,8 +98,7 @@ export class SvelteMap extends Map {
 		var s = sources.get(key);
 
 		if (s === undefined) {
-			var ret = super.get(key);
-			if (ret !== undefined) {
+			if (super.has(key)) {
 				s = this.#source(0);
 
 				if (DEV) {
@@ -134,8 +133,7 @@ export class SvelteMap extends Map {
 		var s = sources.get(key);
 
 		if (s === undefined) {
-			var ret = super.get(key);
-			if (ret !== undefined) {
+			if (super.has(key)) {
 				s = this.#source(0);
 
 				if (DEV) {
@@ -202,8 +200,11 @@ export class SvelteMap extends Map {
 
 		if (s !== undefined) {
 			sources.delete(key);
-			set(this.#size, super.size);
 			set(s, -1);
+		}
+
+		if (res) {
+			set(this.#size, super.size);
 			increment(this.#version);
 		}
 
diff --git a/packages/svelte/src/reactivity/map.test.ts b/packages/svelte/src/reactivity/map.test.ts
index 2f9f064b42..8bb6f72f7b 100644
--- a/packages/svelte/src/reactivity/map.test.ts
+++ b/packages/svelte/src/reactivity/map.test.ts
@@ -207,6 +207,75 @@ test('map handling of undefined values', () => {
 	cleanup();
 });
 
+test('map.has() and map.get() with undefined values', () => {
+	const map = new SvelteMap<string, undefined | string>([['foo', undefined]]);
+
+	const log: any = [];
+
+	const cleanup = effect_root(() => {
+		render_effect(() => {
+			log.push('has', map.has('foo'));
+		});
+
+		render_effect(() => {
+			log.push('get', map.get('foo'));
+		});
+
+		flushSync(() => {
+			map.delete('foo');
+		});
+
+		flushSync(() => {
+			map.set('bar', undefined);
+		});
+	});
+
+	assert.deepEqual(log, [
+		'has',
+		true,
+		'get',
+		undefined,
+		'has',
+		false,
+		'get',
+		undefined,
+		// set('bar') bumps version, causing has('foo')/get('foo') effects to re-run
+		'has',
+		false,
+		'get',
+		undefined
+	]);
+
+	assert.equal(map.has('bar'), true);
+	assert.equal(map.get('bar'), undefined);
+
+	cleanup();
+});
+
+test('map.delete() triggers size reactivity for keys without per-key sources', () => {
+	const map = new SvelteMap([
+		[1, 'a'],
+		[2, 'b']
+	]);
+
+	const log: any = [];
+
+	const cleanup = effect_root(() => {
+		render_effect(() => {
+			log.push(map.size);
+		});
+
+		// delete key 2 which was never individually read (no per-key source)
+		flushSync(() => {
+			map.delete(2);
+		});
+	});
+
+	assert.deepEqual(log, [2, 1]);
+
+	cleanup();
+});
+
 test('not invoking reactivity when value is not in the map after changes', () => {
 	const map = new SvelteMap([[1, 1]]);