--- a/testing/web-platform/README.md
+++ b/testing/web-platform/README.md
@@ -35,17 +35,18 @@ FAQ
   behaviour even if we don't yet pass, get proper review, and have a
   commit message that makes sense outside of the Mozilla
   context. If you are writing tests that should not be upstreamed yet
   for some reason they must be located under
   `testing/web-platform/mozilla/tests`.
 
   It is important to note that in order for the tests to run the
   manifest file must be updated; this should not be done by hand, but
-  by running `mach web-platform-tests --manifest-update`.
+  by running `mach wpt-manifest-update` (or `mach web-platform-tests
+  --manifest-update`, if you also wish to run some tests).
 
   `mach web-platform-tests-create <path>` is a helper script designed
   to help create new web-platform-tests. It opens a locally configured
   editor at `<path>` with web-platform-tests boilerplate filled in,
   and in the background runs `mach web-platform-tests
   --manifest-update <path>`, so the test being developed is added to
   the manifest and opened for interactive development.
 

--- a/testing/web-platform/mach_commands.py
+++ b/testing/web-platform/mach_commands.py
@@ -231,16 +231,37 @@ testing/web-platform/tests for tests tha
             p = create_parser_wpt()
             wpt_kwargs = vars(p.parse_args(["--manifest-update", path]))
             context.commands.dispatch("web-platform-tests", context, **wpt_kwargs)
 
         if proc:
             proc.wait()
 
 
+class WPTManifestUpdater(MozbuildObject):
+    def run_update(self):
+        import imp
+        from wptrunner import wptlogging
+        from wptrunner.wptcommandline import get_test_paths, set_from_config
+        from wptrunner.testloader import ManifestLoader
+
+        wpt_dir = os.path.abspath(os.path.join(self.topsrcdir, 'testing', 'web-platform'))
+
+        localpaths = imp.load_source("localpaths",
+                                     os.path.join(wpt_dir, "tests", "tools", "localpaths.py"))
+        kwargs = {"config": os.path.join(wpt_dir, "wptrunner.ini"),
+                  "tests_root": None,
+                  "metadata_root": None}
+
+        wptlogging.setup({}, {"mach": sys.stdout})
+        set_from_config(kwargs)
+        test_paths = get_test_paths(kwargs["config"])
+        ManifestLoader(test_paths, force_manifest_update=True).load()
+
+
 def create_parser_wpt():
     from wptrunner import wptcommandline
     return wptcommandline.create_parser(["firefox"])
 
 def create_parser_update():
     from update import updatecommandline
     return updatecommandline.create_parser()
 
@@ -317,8 +338,15 @@ class MachCommands(MachCommandBase):
     @Command("web-platform-tests-create",
              category="testing",
              conditions=[conditions.is_firefox],
              parser=create_parser_create)
     def create_web_platform_test(self, **params):
         self.setup()
         wpt_creator = self._spawn(WebPlatformTestsCreator)
         wpt_creator.run_create(self._mach_context, **params)
+
+    @Command("wpt-manifest-update",
+             category="testing")
+    def wpt_manifest_update(self, **parms):
+        self.setup()
+        wpt_manifest_updater = self._spawn(WPTManifestUpdater)
+        wpt_manifest_updater.run_update()