--- a/testing/marionette/harness/marionette/runner/base.py
+++ b/testing/marionette/harness/marionette/runner/base.py
@@ -341,16 +341,17 @@ class BaseMarionetteArguments(ArgumentPa
         self.add_argument('--jsdebugger',
                         action='store_true',
                         default=False,
                         help='Enable the jsdebugger for marionette javascript.')
         self.add_argument('--pydebugger',
                         help='Enable python post-mortem debugger when a test fails.'
                              ' Pass in the debugger you want to use, eg pdb or ipdb.')
         self.add_argument('--socket-timeout',
+                        type=float,
                         default=self.socket_timeout_default,
                         help='Set the global timeout for marionette socket operations.')
         self.add_argument('--disable-e10s',
                         action='store_false',
                         dest='e10s',
                         default=True,
                         help='Disable e10s when running marionette tests.')
         self.add_argument('--tag',

--- a/testing/marionette/harness/marionette/tests/harness_unit/test_marionette_runner.py
+++ b/testing/marionette/harness/marionette/tests/harness_unit/test_marionette_runner.py
@@ -2,16 +2,17 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 import pytest
 from mock import patch, Mock, DEFAULT, mock_open, MagicMock
 
 from marionette.runtests import (
     MarionetteTestRunner,
     MarionetteHarness,
+    MarionetteArguments,
     cli
 )
 from marionette.runner import MarionetteTestResult
 from marionette_driver.marionette import Marionette
 
 # avoid importing MarionetteJSTestCase to prevent pytest from
 # collecting and running it as part of this test suite
 import marionette.marionette_test as marionette_test
@@ -200,16 +201,39 @@ def test_call_harness_with_parsed_args_y
         'marionette.runtests.MarionetteHarness.parse_args'
     ) as parse_args:
         failed_or_crashed = MarionetteHarness(runner_class,
                                               args=mach_parsed_kwargs).run()
         parse_args.assert_not_called()
     assert failed_or_crashed == sum(num_fails_crashed)
 
 
+@pytest.mark.parametrize("sock_timeout_value", ['A', '10', '1B-', '1C2', '44.35'])
+def test_parse_arg_socket_timeout_with_multiple_values(sock_timeout_value):
+    argv = ['marionette', '--socket-timeout', sock_timeout_value]
+    parser = MarionetteArguments()
+
+    def _is_float_convertible(value):
+        try:
+            float(value)
+            return True
+        except:
+            return False
+
+    if not _is_float_convertible(sock_timeout_value):
+        # should raising exception, since sock_timeout must be convertible to float.
+        with pytest.raises(SystemExit) as ex:
+            parser.parse_args(args=argv)
+        assert ex.value.code == 2
+    else:
+        # should pass without raising exception.
+        args = parser.parse_args(args=argv)
+        assert hasattr(args, 'socket_timeout') and args.socket_timeout == float(sock_timeout_value)
+
+
 def test_call_harness_with_no_args_yields_num_failures(runner_class):
     with patch(
         'marionette.runtests.MarionetteHarness.parse_args'
     ) as parse_args:
         parse_args.return_value = {'tests': []}
         failed_or_crashed = MarionetteHarness(runner_class).run()
         assert parse_args.call_count == 1
     assert failed_or_crashed == 0