diff --git a/tests/run-tests b/tests/run-tests
index 3f8a2a8273..4cf16131f5 100755
--- a/tests/run-tests
+++ b/tests/run-tests
@@ -6,6 +6,7 @@ import sys
 import platform
 import argparse
 import re
+import threading
 from glob import glob
 
 # Tests require at least CPython 3.3. If your default python3 executable
@@ -197,13 +198,27 @@ def run_micropython(pyb, args, test_file, is_special=False):
 def run_feature_check(pyb, args, base_path, test_file):
     return run_micropython(pyb, args, base_path + "/feature_check/" + test_file, is_special=True)
 
+class ThreadSafeCounter:
+    def __init__(self, start=0):
+        self._value = start
+        self._lock = threading.Lock()
+
+    def add(self, to_add):
+        with self._lock: self._value += to_add
+
+    def append(self, arg):
+        self.add([arg])
+
+    @property
+    def value(self):
+        return self._value
 
 def run_tests(pyb, tests, args, base_path="."):
-    test_count = 0
-    testcase_count = 0
-    passed_count = 0
-    failed_tests = []
-    skipped_tests = []
+    test_count = ThreadSafeCounter()
+    testcase_count = ThreadSafeCounter()
+    passed_count = ThreadSafeCounter()
+    failed_tests = ThreadSafeCounter([])
+    skipped_tests = ThreadSafeCounter([])
 
     skip_tests = set()
     skip_native = False
@@ -355,8 +370,6 @@ def run_tests(pyb, tests, args, base_path="."):
         skip_tests.add('micropython/schedule.py') # native code doesn't check pending events
 
     def run_one_test(test_file):
-        nonlocal test_count, testcase_count, passed_count, failed_tests
-
         test_file = test_file.replace('\\', '/')
         test_basename = os.path.basename(test_file)
         test_name = os.path.splitext(test_basename)[0]
@@ -417,14 +430,14 @@ def run_tests(pyb, tests, args, base_path="."):
             skipped_tests.append(test_name)
             return
 
-        testcase_count += len(output_expected.splitlines())
+        testcase_count.add(len(output_expected.splitlines()))
 
         filename_expected = test_basename + ".exp"
         filename_mupy = test_basename + ".out"
 
         if output_expected == output_mupy:
             print("pass ", test_file)
-            passed_count += 1
+            passed_count.add(1)
             rm_f(filename_expected)
             rm_f(filename_mupy)
         else:
@@ -439,18 +452,18 @@ def run_tests(pyb, tests, args, base_path="."):
             print("FAIL ", test_file)
             failed_tests.append(test_name)
 
-        test_count += 1
+        test_count.add(1)
 
     for test_file in tests:
         run_one_test(test_file)
 
-    print("{} tests performed ({} individual testcases)".format(test_count, testcase_count))
-    print("{} tests passed".format(passed_count))
+    print("{} tests performed ({} individual testcases)".format(test_count.value, testcase_count.value))
+    print("{} tests passed".format(passed_count.value))
 
-    if len(skipped_tests) > 0:
-        print("{} tests skipped: {}".format(len(skipped_tests), ' '.join(skipped_tests)))
-    if len(failed_tests) > 0:
-        print("{} tests failed: {}".format(len(failed_tests), ' '.join(failed_tests)))
+    if len(skipped_tests.value) > 0:
+        print("{} tests skipped: {}".format(len(skipped_tests.value), ' '.join(skipped_tests.value)))
+    if len(failed_tests.value) > 0:
+        print("{} tests failed: {}".format(len(failed_tests.value), ' '.join(failed_tests.value)))
         return False
 
     # all tests succeeded