diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -172,11 +172,13 @@
 def main():
     # Read config generated by configure.
     config = configparser.ConfigParser()
-    configfile = os.path.abspath(os.path.dirname(__file__)) + "/../config.ini"
+    configfile = os.path.join(os.path.abspath(
+        os.path.dirname(__file__)), "..", "config.ini")
     config.read_file(open(configfile))
 
     src_dir = config["environment"]["SRCDIR"]
-    tests_dir = src_dir + '/test/functional/'
+    build_dir = config["environment"]["BUILDDIR"]
+    tests_dir = os.path.join(src_dir, 'test', 'functional')
 
     # Parse arguments and pass through unrecognised args
     parser = argparse.ArgumentParser(add_help=False,
@@ -204,7 +206,8 @@
     parser.add_argument('--tmpdirprefix', '-t',
                         default=tempfile.gettempdir(), help="Root directory for datadirs")
     parser.add_argument('--junitouput', '-ju',
-                        default=tests_dir + 'junit_results.xml', help="file that will store JUnit formated test results ")
+                        default=os.path.join(build_dir, 'junit_results.xml'), help="file that will store JUnit formated test results.")
+
     args, unknown_args = parser.parse_known_args()
 
     # Create a set to store arguments and create the passon string
@@ -217,7 +220,7 @@
     logging.basicConfig(format='%(message)s', level=logging_level)
 
     # Create base test directory
-    tmpdir = "%s/bitcoin_test_runner_%s" % (
+    tmpdir = os.path.join("%s", "bitcoin_test_runner_%s") % (
         args.tmpdirprefix, datetime.datetime.now().strftime("%Y%m%d_%H%M%S"))
     os.makedirs(tmpdir)
 
@@ -274,16 +277,16 @@
         # and exit.
         parser.print_help()
         subprocess.check_call(
-            (tests_dir + test_list[0]).split() + ['-h'])
+            [os.path.join(tests_dir, test_list[0]), '-h'])
         sys.exit(0)
 
     check_script_list(src_dir)
 
     if not args.keepcache:
-        shutil.rmtree("%s/test/cache" %
-                      config["environment"]["BUILDDIR"], ignore_errors=True)
+        shutil.rmtree(os.path.join(build_dir, "test",
+                                   "cache"), ignore_errors=True)
 
-    run_tests(test_list, src_dir, config["environment"]["BUILDDIR"], tests_dir, args.junitouput,
+    run_tests(test_list, src_dir, build_dir, tests_dir, args.junitouput,
               config["environment"]["EXEEXT"], tmpdir, args.jobs, args.coverage, passon_args)
 
 
@@ -298,17 +301,19 @@
         pass
 
     # Warn if there is a cache directory
-    cache_dir = "%s/test/cache" % build_dir
+    cache_dir = os.path.join(build_dir, "test", "cache")
     if os.path.isdir(cache_dir):
         print("%sWARNING!%s There is a cache directory here: %s. If tests fail unexpectedly, try deleting the cache directory." % (
             BOLD[1], BOLD[0], cache_dir))
 
     # Set env vars
     if "BITCOIND" not in os.environ:
-        os.environ["BITCOIND"] = build_dir + '/src/bitcoind' + exeext
-        os.environ["BITCOINCLI"] = build_dir + '/src/bitcoin-cli' + exeext
+        os.environ["BITCOIND"] = os.path.join(
+            build_dir, 'src', 'bitcoind' + exeext)
+        os.environ["BITCOINCLI"] = os.path.join(
+            build_dir, 'src', 'bitcoin-cli' + exeext)
 
-    flags = ["--srcdir={}/src".format(build_dir)] + args
+    flags = [os.path.join("--srcdir={}".format(build_dir), "src")] + args
     flags.append("--cachedir=%s" % cache_dir)
 
     if enable_coverage:
@@ -321,7 +326,7 @@
     if len(test_list) > 1 and jobs > 1:
         # Populate cache
         subprocess.check_output(
-            [tests_dir + 'create_cache.py'] + flags + ["--tmpdir=%s/cache" % tmpdir])
+            [os.path.join(tests_dir, 'create_cache.py')] + flags + [os.path.join("--tmpdir=%s", "cache") % tmpdir])
 
     # Run Tests
     job_queue = TestHandler(jobs, tests_dir, tmpdir, test_list, flags)
@@ -417,11 +422,11 @@
             log_stdout = tempfile.SpooledTemporaryFile(max_size=2**16)
             log_stderr = tempfile.SpooledTemporaryFile(max_size=2**16)
             test_argv = t.split()
-            tmpdir = ["--tmpdir=%s/%s_%s" %
-                      (self.tmpdir, re.sub(".py$", "", test_argv[0]), portseed)]
+            tmpdir = [os.path.join("--tmpdir=%s", "%s_%s") %
+                      (self.tmpdir, re.sub(".py$", "", t), portseed)]
             self.jobs.append((t,
                               time.time(),
-                              subprocess.Popen([self.tests_dir + test_argv[0]] + test_argv[1:] + self.flags + portseed_arg + tmpdir,
+                              subprocess.Popen([os.path.join(self.tests_dir, test_argv[0])] + test_argv[1:] + self.flags + portseed_arg + tmpdir,
                                                universal_newlines=True,
                                                stdout=log_stdout,
                                                stderr=log_stderr),
@@ -484,7 +489,7 @@
 
     Check that there are no scripts in the functional tests directory which are
     not being run by pull-tester.py."""
-    script_dir = src_dir + '/test/functional/'
+    script_dir = os.path.join(src_dir, 'test', 'functional')
     python_files = set([t for t in os.listdir(script_dir) if t[-3:] == ".py"])
     missed_tests = list(
         python_files - set(map(lambda x: x.split()[0], ALL_SCRIPTS + NON_SCRIPTS)))