diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0369f54ac8c..669a1531fb2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,15 @@
 
 [pub-security-advisories]: https://dart.dev/go/pub-security-advisories
 
+### Libraries
+
+#### `dart:js_interop`
+
+- On dart2wasm, `JSBoxedDartObject` now is an actual JS object that wraps the
+  opaque Dart value instead of only externalizing the value. Like the JS
+  backends, you'll now get a more useful error when trying to use it in another
+  Dart runtime.
+
 ## 3.3.0
 
 ### Language
diff --git a/sdk/lib/_internal/wasm/lib/js_interop_patch.dart b/sdk/lib/_internal/wasm/lib/js_interop_patch.dart
index aef57c95003..8f9c2926d8a 100644
--- a/sdk/lib/_internal/wasm/lib/js_interop_patch.dart
+++ b/sdk/lib/_internal/wasm/lib/js_interop_patch.dart
@@ -94,11 +94,27 @@ extension FunctionToJSExportedDartFunction on Function {
       'by the interop transformer.');
 }
 
+/// Embedded global property for wrapped Dart objects passed via JS interop.
+///
+/// This is a Symbol so that different Dart applications don't share Dart
+/// objects from different Dart runtimes. We expect all [JSBoxedDartObject]s to
+/// have this Symbol.
+final JSSymbol _jsBoxedDartObjectProperty = _boxNonNullable<JSSymbol>(
+    js_helper.JS<WasmExternRef?>('() => Symbol("jsBoxedDartObjectProperty")'));
+
 /// [JSBoxedDartObject] <-> [Object]
 @patch
 extension JSBoxedDartObjectToObject on JSBoxedDartObject {
   @patch
-  Object get toDart => jsObjectToDartObject(toExternRef);
+  Object get toDart {
+    final val = js_helper.JS<WasmExternRef?>('(o,s) => o[s]', this.toExternRef,
+        _jsBoxedDartObjectProperty.toExternRef);
+    if (isDartNull(val)) {
+      throw 'Expected a wrapped Dart object, but got a JS object or a wrapped '
+          'Dart object from a separate runtime instead.';
+    }
+    return jsObjectToDartObject(val);
+  }
 }
 
 @patch
@@ -108,7 +124,10 @@ extension ObjectToJSBoxedDartObject on Object {
     if (this is JSValue) {
       throw 'Attempting to box non-Dart object.';
     }
-    return _boxNonNullable<JSBoxedDartObject>(jsObjectFromDartObject(this));
+    final box = JSObject();
+    js_helper.JS<WasmExternRef?>('(o,s,v) => o[s] = v', box.toExternRef,
+        _jsBoxedDartObjectProperty.toExternRef, jsObjectFromDartObject(this));
+    return box as JSBoxedDartObject;
   }
 }
 
diff --git a/tests/lib/js/static_interop_test/js_types_test.dart b/tests/lib/js/static_interop_test/js_types_test.dart
index e6da910af27..666d3855bcd 100644
--- a/tests/lib/js/static_interop_test/js_types_test.dart
+++ b/tests/lib/js/static_interop_test/js_types_test.dart
@@ -173,6 +173,7 @@ void syncTests() {
   expect(edo is JSBoxedDartObject, true);
   expect(confuse(edo) is JSBoxedDartObject, true);
   expect(((edo as JSBoxedDartObject).toDart as DartObject).foo, 'bar');
+  expect(edo.instanceOfString('Object'), true);
   // Functions should be boxed without assertInterop.
   final concat = (String a, String b) => a + b;
   edo = concat.toJSBox;