--- a/testing/mochitest/mochitest_options.py
+++ b/testing/mochitest/mochitest_options.py
@@ -525,17 +525,17 @@ class MochitestArguments(ArgumentContain
           }],
         [["--valgrind"],
          {"default": None,
           "help": "Valgrind binary to run tests with. Program name or path.",
           }],
         [["--valgrind-args"],
          {"dest": "valgrindArgs",
           "default": None,
-          "help": "Extra arguments to pass to Valgrind.",
+          "help": "Comma-separated list of extra arguments to pass to Valgrind.",
           }],
         [["--valgrind-supp-files"],
          {"dest": "valgrindSuppFiles",
           "default": None,
           "help": "Comma-separated list of suppression files to pass to Valgrind.",
           }],
         [["--debugger-interactive"],
          {"action": "store_true",

--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -1883,17 +1883,17 @@ class MochitestDesktop(MochitestBase):
         if debuggerInfo:
             interactive = debuggerInfo.interactive
             debug_args = [debuggerInfo.path] + debuggerInfo.args
 
         # Set up Valgrind arguments.
         if valgrindPath:
             interactive = False
             valgrindArgs_split = ([] if valgrindArgs is None
-                                  else valgrindArgs.split())
+                                  else valgrindArgs.split(","))
 
             valgrindSuppFiles_final = []
             if valgrindSuppFiles is not None:
                 valgrindSuppFiles_final = ["--suppressions=" + path for path in valgrindSuppFiles.split(",")]
 
             debug_args = ([valgrindPath]
                           + mozdebug.get_default_valgrind_args()
                           + valgrindArgs_split