diff --git a/tests/std/tests/P1502R1_standard_library_header_units/lit.local.cfg b/tests/std/tests/P1502R1_standard_library_header_units/lit.local.cfg
index a58f404ef47..5f1221f3b0f 100644
--- a/tests/std/tests/P1502R1_standard_library_header_units/lit.local.cfg
+++ b/tests/std/tests/P1502R1_standard_library_header_units/lit.local.cfg
@@ -7,3 +7,4 @@ site.addsitedir(os.path.dirname(os.path.dirname(__file__)))
 import P1502R1_standard_library_header_units.custom_format
 
 config.test_format = P1502R1_standard_library_header_units.custom_format.CustomTestFormat()
+config.parallelism_group = 'multi-threaded'
diff --git a/tests/utils/stl/test/config.py b/tests/utils/stl/test/config.py
index 5cff67b5133..d18d8170f38 100644
--- a/tests/utils/stl/test/config.py
+++ b/tests/utils/stl/test/config.py
@@ -46,3 +46,6 @@ def configure(parameters, features, config, lit_config):
     ['-LIBPATH:' + os.path.normpath(dir) for dir in lit_config.library_dirs[config.name]]
 
   lit_config.test_env = {'PATH' : os.path.normpath(lit_config.cxx_runtime)}
+
+  # Add a parallelism group that multi-threaded tests can be marked as a part of.
+  lit_config.parallelism_groups['multi-threaded'] = 1