diff --git a/testing/marionette/evaluate.js b/testing/marionette/evaluate.js
index ac781530e723..20fada29e58f 100644
--- a/testing/marionette/evaluate.js
+++ b/testing/marionette/evaluate.js
@@ -247,7 +247,7 @@ evaluate.fromJSON = function (obj, seenEls, win, shadowRoot = undefined) {
  *     web elements.
  */
 evaluate.toJSON = function (obj, seenEls) {
-  let t = Object.prototype.toString.call(obj);
+  const t = Object.prototype.toString.call(obj);
 
   // null
   if (t == "[object Undefined]" || t == "[object Null]") {
@@ -270,6 +270,12 @@ evaluate.toJSON = function (obj, seenEls) {
     return element.makeWebElement(uuid);
   }
 
+  // custom JSON representation
+  else if (typeof obj["toJSON"] == "function") {
+    let unsafeJSON = obj.toJSON();
+    return evaluate.toJSON(unsafeJSON, seenEls);
+  }
+
   // arbitrary objects + files
   else {
     let rv = {};
diff --git a/testing/marionette/harness/marionette_harness/tests/unit/test_execute_script.py b/testing/marionette/harness/marionette_harness/tests/unit/test_execute_script.py
index 66054376d622..1a7c208c5566 100644
--- a/testing/marionette/harness/marionette_harness/tests/unit/test_execute_script.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_execute_script.py
@@ -49,6 +49,9 @@ class TestExecuteContent(MarionetteTestCase):
             "return typeof arguments[0] != 'undefined'", [property], sandbox=sandbox),
             "property {} is undefined".format(property))
 
+    def assert_is_web_element(self, element):
+        self.assertIsInstance(element, HTMLElement)
+
     def test_return_number(self):
         self.assertEqual(1, self.marionette.execute_script("return 1"))
         self.assertEqual(1.5, self.marionette.execute_script("return 1.5"))
@@ -330,6 +333,25 @@ class TestExecuteContent(MarionetteTestCase):
         # test inspection of arguments
         self.marionette.execute_script("__webDriverArguments.toString()")
 
+    def test_toJSON(self):
+        foo = self.marionette.execute_script("""
+            return {
+              toJSON () {
+                return "foo";
+              }
+            }""",
+            sandbox=None)
+        self.assertEqual("foo", foo)
+
+    def test_unsafe_toJSON(self):
+        el = self.marionette.execute_script("""
+            return {
+              toJSON () {
+                return document.documentElement;
+              }
+            }""",
+            sandbox=None)
+        self.assert_is_web_element(el)
 
 
 class TestExecuteChrome(WindowManagerMixin, TestExecuteContent):