diff --git a/testing/mozharness/scripts/android_emulator_unittest.py b/testing/mozharness/scripts/android_emulator_unittest.py
index be73074109bc..c6329db29ebc 100644
--- a/testing/mozharness/scripts/android_emulator_unittest.py
+++ b/testing/mozharness/scripts/android_emulator_unittest.py
@@ -463,23 +463,28 @@ class AndroidEmulatorTest(BlobUploadMixin, TestingMixin, EmulatorMixin, VCSMixin
                 self.config.get('marionette_test_manifest', '')
             ),
         }
+
+        user_paths = os.environ.get('MOZHARNESS_TEST_PATHS')
         for option in self.config["suite_definitions"][self.test_suite]["options"]:
             opt = option.split('=')[0]
             # override configured chunk options with script args, if specified
-            if opt == '--this-chunk' and self.this_chunk is not None:
-                continue
-            if opt == '--total-chunks' and self.total_chunks is not None:
-                continue
+            if opt in ('--this-chunk', '--total-chunks'):
+                if user_paths or getattr(self, opt.replace('-', '_').strip('_'), None) is not None:
+                    continue
+
             if '%(app)' in option:
                 # only query package name if requested
                 cmd.extend([option % {'app': self._query_package_name()}])
             else:
                 cmd.extend([option % str_format_values])
 
-        if self.this_chunk is not None:
-            cmd.extend(['--this-chunk', self.this_chunk])
-        if self.total_chunks is not None:
-            cmd.extend(['--total-chunks', self.total_chunks])
+        if user_paths:
+            cmd.extend(user_paths.split(':'))
+        else:
+            if self.this_chunk is not None:
+                cmd.extend(['--this-chunk', self.this_chunk])
+            if self.total_chunks is not None:
+                cmd.extend(['--total-chunks', self.total_chunks])
 
         try_options, try_tests = self.try_args(self.test_suite)
         cmd.extend(try_options)
diff --git a/testing/mozharness/scripts/desktop_unittest.py b/testing/mozharness/scripts/desktop_unittest.py
index 5e067c78036b..91eeb8bfae05 100755
--- a/testing/mozharness/scripts/desktop_unittest.py
+++ b/testing/mozharness/scripts/desktop_unittest.py
@@ -415,7 +415,10 @@ class DesktopUnittest(TestingMixin, MercurialScript, BlobUploadMixin, MozbaseMix
                 elif suite_category not in SUITE_DEFAULT_E10S and c['e10s']:
                     base_cmd.append('--e10s')
 
-            if c.get('total_chunks') and c.get('this_chunk'):
+            # Ignore chunking if we have user specified test paths
+            if os.environ.get('MOZHARNESS_TEST_PATHS'):
+                base_cmd.extend(os.environ['MOZHARNESS_TEST_PATHS'].split(':'))
+            elif c.get('total_chunks') and c.get('this_chunk'):
                 base_cmd.extend(['--total-chunks', c['total_chunks'],
                                  '--this-chunk', c['this_chunk']])
 
diff --git a/testing/mozharness/scripts/marionette.py b/testing/mozharness/scripts/marionette.py
index 54d5c76c66a1..303a01f77617 100755
--- a/testing/mozharness/scripts/marionette.py
+++ b/testing/mozharness/scripts/marionette.py
@@ -312,7 +312,10 @@ class MarionetteTest(TestingMixin, MercurialScript, BlobUploadMixin, TransferMix
             # Make sure that the logging directory exists
             self.fatal("Could not create blobber upload directory")
 
-        cmd.append(manifest)
+        if os.environ.get('MOZHARNESS_TEST_PATHS'):
+            cmd.extend(os.environ['MOZHARNESS_TEST_PATHS'].split(':'))
+        else:
+            cmd.append(manifest)
 
         try_options, try_tests = self.try_args("marionette")
         cmd.extend(self.query_tests_args(try_tests,
diff --git a/testing/mozharness/scripts/web_platform_tests.py b/testing/mozharness/scripts/web_platform_tests.py
index c9f51dfac81a..98b92e4ce84b 100755
--- a/testing/mozharness/scripts/web_platform_tests.py
+++ b/testing/mozharness/scripts/web_platform_tests.py
@@ -224,10 +224,17 @@ class WebPlatformTest(TestingMixin, MercurialScript, BlobUploadMixin, CodeCovera
         else:
             cmd.append("--stylo-threads=4")
 
-        for opt in ["total_chunks", "this_chunk"]:
-            val = c.get(opt)
-            if val:
-                cmd.append("--%s=%s" % (opt.replace("_", "-"), val))
+        if os.environ.get('MOZHARNESS_TEST_PATHS'):
+            prefix = 'testing/web-platform'
+            paths = os.environ['MOZHARNESS_TEST_PATHS'].split(':')
+            paths = [os.path.join(dirs["abs_wpttest_dir"], os.path.relpath(p, prefix))
+                     for p in paths if p.startswith(prefix)]
+            cmd.extend(paths)
+        else:
+            for opt in ["total_chunks", "this_chunk"]:
+                val = c.get(opt)
+                if val:
+                    cmd.append("--%s=%s" % (opt.replace("_", "-"), val))
 
         if "wdspec" in test_types:
             geckodriver_path = self._query_geckodriver()