diff --git a/src/browser/tests/window/onerror.html b/src/browser/tests/window/onerror.html
new file mode 100644
index 00000000..d07ba190
--- /dev/null
+++ b/src/browser/tests/window/onerror.html
@@ -0,0 +1,211 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+
+<script id=onerrorBasicCallback>
+{
+  let callbackCalled = false;
+  let receivedArgs = null;
+
+  window.onerror = function(message, source, lineno, colno, error) {
+    callbackCalled = true;
+    receivedArgs = { message, source, lineno, colno, error };
+  };
+
+  const err = new Error('Test error');
+  window.reportError(err);
+
+  testing.expectEqual(true, callbackCalled);
+  testing.expectEqual(true, receivedArgs.message.includes('Test error'));
+  testing.expectEqual(err, receivedArgs.error);
+
+  window.onerror = null;
+}
+</script>
+
+<script id=onerrorFiveArguments>
+{
+  let argCount = 0;
+
+  window.onerror = function() {
+    argCount = arguments.length;
+  };
+
+  window.reportError(new Error('Five args test'));
+
+  // Per WHATWG spec, onerror receives exactly 5 arguments
+  testing.expectEqual(5, argCount);
+
+  window.onerror = null;
+}
+</script>
+
+<script id=onerrorReturnTrueCancelsEvent>
+{
+  let listenerCalled = false;
+
+  window.onerror = function() {
+    return true; // Should cancel the event
+  };
+
+  const listener = function() {
+    listenerCalled = true;
+  };
+  window.addEventListener('error', listener);
+
+  window.reportError(new Error('Should be cancelled'));
+
+  // The event listener should still be called (onerror returning true
+  // only prevents default, not propagation)
+  testing.expectEqual(true, listenerCalled);
+
+  window.onerror = null;
+  window.removeEventListener('error', listener);
+}
+</script>
+
+<script id=onerrorAndEventListenerBothCalled>
+{
+  let onerrorCalled = false;
+  let listenerCalled = false;
+
+  window.onerror = function() {
+    onerrorCalled = true;
+  };
+
+  const listener = function() {
+    listenerCalled = true;
+  };
+  window.addEventListener('error', listener);
+
+  window.reportError(new Error('Both should fire'));
+
+  testing.expectEqual(true, onerrorCalled);
+  testing.expectEqual(true, listenerCalled);
+
+  window.onerror = null;
+  window.removeEventListener('error', listener);
+}
+</script>
+
+<script id=onerrorCalledBeforeEventListener>
+{
+  let callOrder = [];
+
+  window.onerror = function() {
+    callOrder.push('onerror');
+  };
+
+  const listener = function() {
+    callOrder.push('listener');
+  };
+  window.addEventListener('error', listener);
+
+  window.reportError(new Error('Order test'));
+
+  // onerror should be called before addEventListener handlers
+  testing.expectEqual('onerror', callOrder[0]);
+  testing.expectEqual('listener', callOrder[1]);
+
+  window.onerror = null;
+  window.removeEventListener('error', listener);
+}
+</script>
+
+<script id=onerrorGetterSetter>
+{
+  const handler = function() {};
+
+  testing.expectEqual(null, window.onerror);
+
+  window.onerror = handler;
+  testing.expectEqual(handler, window.onerror);
+
+  window.onerror = null;
+  testing.expectEqual(null, window.onerror);
+}
+</script>
+
+<script id=onerrorWithNonFunction>
+{
+  // Setting onerror to a non-function should not throw
+  // but should not be stored as the handler
+  window.onerror = "not a function";
+  testing.expectEqual(null, window.onerror);
+
+  window.onerror = {};
+  testing.expectEqual(null, window.onerror);
+
+  window.onerror = 123;
+  testing.expectEqual(null, window.onerror);
+}
+</script>
+
+<script id=onerrorArgumentTypes>
+{
+  let receivedTypes = null;
+
+  window.onerror = function(message, source, lineno, colno, error) {
+    receivedTypes = {
+      message: typeof message,
+      source: typeof source,
+      lineno: typeof lineno,
+      colno: typeof colno,
+      error: typeof error
+    };
+  };
+
+  window.reportError(new Error('Type check'));
+
+  testing.expectEqual('string', receivedTypes.message);
+  testing.expectEqual('string', receivedTypes.source);
+  testing.expectEqual('number', receivedTypes.lineno);
+  testing.expectEqual('number', receivedTypes.colno);
+  testing.expectEqual('object', receivedTypes.error);
+
+  window.onerror = null;
+}
+</script>
+
+<script id=onerrorReturnFalseDoesNotCancel>
+{
+  let eventDefaultPrevented = false;
+
+  window.onerror = function() {
+    return false; // Should NOT cancel the event
+  };
+
+  const listener = function(e) {
+    eventDefaultPrevented = e.defaultPrevented;
+  };
+  window.addEventListener('error', listener);
+
+  window.reportError(new Error('Return false test'));
+
+  testing.expectEqual(false, eventDefaultPrevented);
+
+  window.onerror = null;
+  window.removeEventListener('error', listener);
+}
+</script>
+
+<script id=onerrorReturnTruePreventsDefault>
+{
+  let eventDefaultPrevented = false;
+
+  window.onerror = function() {
+    return true; // Should cancel (prevent default)
+  };
+
+  const listener = function(e) {
+    eventDefaultPrevented = e.defaultPrevented;
+  };
+  window.addEventListener('error', listener);
+
+  window.reportError(new Error('Return true test'));
+
+  testing.expectEqual(true, eventDefaultPrevented);
+
+  window.onerror = null;
+  window.removeEventListener('error', listener);
+}
+</script>
diff --git a/src/browser/webapi/Window.zig b/src/browser/webapi/Window.zig
index 781081c4..7434ab75 100644
--- a/src/browser/webapi/Window.zig
+++ b/src/browser/webapi/Window.zig
@@ -58,7 +58,7 @@ _storage_bucket: *storage.Bucket,
 _on_load: ?js.Function.Global = null,
 _on_pageshow: ?js.Function.Global = null,
 _on_popstate: ?js.Function.Global = null,
-_on_error: ?js.Function.Global = null, // TODO: invoke on error?
+_on_error: ?js.Function.Global = null,
 _on_unhandled_rejection: ?js.Function.Global = null, // TODO: invoke on error
 _location: *Location,
 _timer_id: u30 = 0,
@@ -283,6 +283,32 @@ pub fn reportError(self: *Window, err: js.Value, page: *Page) !void {
     }, page);
 
     const event = error_event.asEvent();
+
+    // Invoke window.onerror callback if set (per WHATWG spec, this is called
+    // with 5 arguments: message, source, lineno, colno, error)
+    // If it returns true, the event is cancelled.
+    if (self._on_error) |on_error| {
+        var ls: js.Local.Scope = undefined;
+        page.js.localScope(&ls);
+        defer ls.deinit();
+
+        const local_func = ls.toLocal(on_error);
+        const result = local_func.call(js.Value, .{
+            error_event._message,
+            error_event._filename,
+            error_event._line_number,
+            error_event._column_number,
+            err,
+        }) catch null;
+
+        // Per spec: returning true from onerror cancels the event
+        if (result) |r| {
+            if (r.isTrue()) {
+                event._prevent_default = true;
+            }
+        }
+    }
+
     try page._event_manager.dispatch(self.asEventTarget(), event);
 
     if (comptime builtin.is_test == false) {
@@ -665,7 +691,7 @@ pub const JsApi = struct {
     pub const onload = bridge.accessor(Window.getOnLoad, Window.setOnLoad, .{});
     pub const onpageshow = bridge.accessor(Window.getOnPageShow, Window.setOnPageShow, .{});
     pub const onpopstate = bridge.accessor(Window.getOnPopState, Window.setOnPopState, .{});
-    pub const onerror = bridge.accessor(Window.getOnError, Window.getOnError, .{});
+    pub const onerror = bridge.accessor(Window.getOnError, Window.setOnError, .{});
     pub const onunhandledrejection = bridge.accessor(Window.getOnUnhandledRejection, Window.setOnUnhandledRejection, .{});
     pub const fetch = bridge.function(Window.fetch, .{});
     pub const queueMicrotask = bridge.function(Window.queueMicrotask, .{});
