diff --git a/.changeset/snapshot-own-proto-key.md b/.changeset/snapshot-own-proto-key.md
new file mode 100644
index 0000000000..9d99362c1e
--- /dev/null
+++ b/.changeset/snapshot-own-proto-key.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep an own `__proto__` key in `$state.snapshot`
diff --git a/packages/svelte/src/internal/shared/clone.js b/packages/svelte/src/internal/shared/clone.js
index ba0c731047..11f02d1a24 100644
--- a/packages/svelte/src/internal/shared/clone.js
+++ b/packages/svelte/src/internal/shared/clone.js
@@ -1,7 +1,7 @@
 /** @import { Snapshot } from './types' */
 import { DEV } from 'esm-env';
 import * as w from './warnings.js';
-import { get_prototype_of, is_array, object_prototype } from './utils.js';
+import { define_property, get_prototype_of, is_array, object_prototype } from './utils.js';
 
 /**
  * In dev, we keep track of which properties could not be cloned. In prod
@@ -90,7 +90,7 @@ function clone(value, cloned, path, paths, original = null, no_tojson = false) {
 			}
 
 			for (var key of Object.keys(value)) {
-				copy[key] = clone(
+				var cloned_value = clone(
 					// @ts-expect-error
 					value[key],
 					cloned,
@@ -99,6 +99,22 @@ function clone(value, cloned, path, paths, original = null, no_tojson = false) {
 					null,
 					no_tojson
 				);
+
+				if (key === '__proto__') {
+					// Assigning `__proto__` runs the setter inherited from `Object.prototype`
+					// rather than creating a property, so the key would be dropped and an
+					// object value would become the copy's prototype, leaving the snapshot
+					// inheriting fields that were data. `structuredClone`, which this
+					// function falls back to below, keeps it as an own property.
+					define_property(copy, key, {
+						value: cloned_value,
+						writable: true,
+						enumerable: true,
+						configurable: true
+					});
+				} else {
+					copy[key] = cloned_value;
+				}
 			}
 
 			return copy;
diff --git a/packages/svelte/tests/runtime-runes/samples/state-snapshot-proto-key/_config.js b/packages/svelte/tests/runtime-runes/samples/state-snapshot-proto-key/_config.js
new file mode 100644
index 0000000000..d589156c07
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/state-snapshot-proto-key/_config.js
@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `<div>["__proto__","b"]</div><div>true</div><div></div>`
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/state-snapshot-proto-key/main.svelte b/packages/svelte/tests/runtime-runes/samples/state-snapshot-proto-key/main.svelte
new file mode 100644
index 0000000000..6e95cf2eca
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/state-snapshot-proto-key/main.svelte
@@ -0,0 +1,11 @@
+<script>
+	// An own `__proto__` key comes from JSON, which is where state hydrated
+	// from a response, from storage, or from a query string comes from.
+	let state = $state(JSON.parse('{"__proto__":{"admin":true},"b":2}'));
+
+	const snapshot = $state.snapshot(state);
+</script>
+
+<div>{JSON.stringify(Object.keys(snapshot))}</div>
+<div>{Object.getPrototypeOf(snapshot) === Object.prototype}</div>
+<div>{snapshot.admin}</div>