--- a/taskcluster/mach_commands.py
+++ b/taskcluster/mach_commands.py
@@ -36,17 +36,17 @@ class ShowTaskGraphSubCommand(SubCommand
             CommandArgument('--verbose', '-v', action="store_true",
                             help="include debug-level logging output"),
             CommandArgument('--json', '-J', action="store_const",
                             dest="format", const="json",
                             help="Output task graph as a JSON object"),
             CommandArgument('--labels', '-L', action="store_const",
                             dest="format", const="labels",
                             help="Output the label for each task in the task graph (default)"),
-            CommandArgument('--parameters', '-p', required=True,
+            CommandArgument('--parameters', '-p', default="project=mozilla-central",
                             help="parameters file (.yml or .json; see "
                                  "`taskcluster/docs/parameters.rst`)`"),
             CommandArgument('--no-optimize', dest="optimize", action="store_false",
                             default="true",
                             help="do not remove tasks from the graph that are found in the "
                             "index (a.k.a. optimize the graph)"),
             CommandArgument('--tasks-regex', '--tasks', default=None,
                             help="only return tasks with labels matching this regular "
@@ -334,17 +334,17 @@ class MachCommands(MachCommandBase):
 
     def show_taskgraph(self, graph_attr, options):
         import taskgraph.parameters
         import taskgraph.target_tasks
         import taskgraph.generator
 
         try:
             self.setup_logging(quiet=options['quiet'], verbose=options['verbose'])
-            parameters = taskgraph.parameters.load_parameters_file(options)
+            parameters = taskgraph.parameters.load_parameters_file(options['parameters'])
             parameters.check()
 
             tgg = taskgraph.generator.TaskGraphGenerator(
                 root_dir=options['root'],
                 parameters=parameters)
 
             tg = getattr(tgg, graph_attr)
 

--- a/taskcluster/taskgraph/parameters.py
+++ b/taskcluster/taskgraph/parameters.py
@@ -52,35 +52,43 @@ class Parameters(ReadOnlyDict):
         if k not in PARAMETER_NAMES:
             raise KeyError("no such parameter {!r}".format(k))
         try:
             return super(Parameters, self).__getitem__(k)
         except KeyError:
             raise KeyError("taskgraph parameter {!r} not found".format(k))
 
 
-def load_parameters_file(options):
+def load_parameters_file(filename):
     """
-    Load parameters from the --parameters option
+    Load parameters from a path, url, decision task-id or project.
+
+    Examples:
+        task-id=fdtgsD5DQUmAQZEaGMvQ4Q
+        project=mozilla-central
     """
     import urllib
-    from taskgraph.util.taskcluster import get_artifact_url
-
-    filename = options['parameters']
+    from taskgraph.util.taskcluster import get_artifact_url, find_task_id
 
     if not filename:
         return Parameters()
 
     try:
         # reading parameters from a local parameters.yml file
         f = open(filename)
     except IOError:
-        # fetching parameters.yml using task task-id or supplied url
+        # fetching parameters.yml using task task-id, project or supplied url
+        task_id = None
         if filename.startswith("task-id="):
             task_id = filename.split("=")[1]
+        elif filename.startswith("project="):
+            index = "gecko.v2.{}.latest.firefox.decision".format(filename.split("=")[1])
+            task_id = find_task_id(index)
+
+        if task_id:
             filename = get_artifact_url(task_id, 'public/parameters.yml')
         f = urllib.urlopen(filename)
 
     if filename.endswith('.yml'):
         return Parameters(**yaml.safe_load(f))
     elif filename.endswith('.json'):
         return Parameters(**json.load(f))
     else:

--- a/taskcluster/taskgraph/test/test_parameters.py
+++ b/taskcluster/taskgraph/test/test_parameters.py
@@ -44,20 +44,20 @@ class TestParameters(unittest.TestCase):
 
     def test_Parameters_check_extra(self):
         p = Parameters(xyz=10, **self.vals)
         self.assertRaises(Exception, lambda: p.check())
 
     def test_load_parameters_file_yaml(self):
         with MockedOpen({"params.yml": "some: data\n"}):
             self.assertEqual(
-                    load_parameters_file({'parameters': 'params.yml'}),
+                    load_parameters_file('params.yml'),
                     {'some': 'data'})
 
     def test_load_parameters_file_json(self):
         with MockedOpen({"params.json": '{"some": "data"}'}):
             self.assertEqual(
-                    load_parameters_file({'parameters': 'params.json'}),
+                    load_parameters_file('params.json'),
                     {'some': 'data'})
 
 
 if __name__ == '__main__':
     main()