diff --git a/ports/webassembly/objjsproxy.c b/ports/webassembly/objjsproxy.c
index cbfe8be49baf..167d4382bc63 100644
--- a/ports/webassembly/objjsproxy.c
+++ b/ports/webassembly/objjsproxy.c
@@ -46,8 +46,14 @@ EM_JS(bool, has_attr, (int jsref, const char *str), {
 EM_JS(bool, lookup_attr, (int jsref, const char *str, uint32_t * out), {
     const base = proxy_js_ref[jsref];
     const attr = UTF8ToString(str);
-    if (attr in base) {
-        let value = base[attr];
+
+    // Attempt to lookup the requested attribute from the base object:
+    // - If the value is not `undefined` then the attribute exists with that value.
+    // - Otherwise if the value is `undefined` and the `in` operator returns true, then
+    //   that attribute does exist and is intended to have a value of `undefined`.
+    // - Otherwise, the attribute does not exist.
+    let value = base[attr];
+    if (value !== undefined || attr in base) {
         if (typeof value === "function") {
             if (base !== globalThis) {
                 if ("_ref" in value) {
diff --git a/tests/ports/webassembly/js_proxy_attribute.mjs b/tests/ports/webassembly/js_proxy_attribute.mjs
new file mode 100644
index 000000000000..5d950122778b
--- /dev/null
+++ b/tests/ports/webassembly/js_proxy_attribute.mjs
@@ -0,0 +1,34 @@
+// Test lookup of attributes on JsProxy objects.
+
+const mp = await (await import(process.argv[2])).loadMicroPython();
+
+// Simple attribute names and values.
+globalThis.obj1 = { a: 1, b: 2 };
+
+// Unconventional attribute names and values.
+globalThis.obj2 = { undefined: "undefined", undef: undefined };
+
+// Dynamically created attribute names and values.
+globalThis.obj3 = new Proxy(new Map(), {
+    get(map, name) {
+        if (!map.has(name)) {
+            console.log("creating attribute", name);
+            map.set(name, name);
+        }
+        return map.get(name);
+    },
+});
+
+mp.runPython(`
+import js
+
+print(js.obj1.a, js.obj1.b)
+print(js.obj1["a"], js.obj1["b"])
+
+print(js.obj2.undefined, js.obj2.undef)
+
+print(js.obj3.c)
+print(js.obj3["c"])
+print(hasattr(js.obj3, "d"))
+print(js.obj3.d)
+`);
diff --git a/tests/ports/webassembly/js_proxy_attribute.mjs.exp b/tests/ports/webassembly/js_proxy_attribute.mjs.exp
new file mode 100644
index 000000000000..7de85a56648a
--- /dev/null
+++ b/tests/ports/webassembly/js_proxy_attribute.mjs.exp
@@ -0,0 +1,9 @@
+1 2
+1 2
+undefined <undefined>
+creating attribute c
+c
+c
+creating attribute d
+True
+d