diff --git a/cipd b/cipd
index f7f4133ac..0de9aea45 100755
--- a/cipd
+++ b/cipd
@@ -6,6 +6,12 @@
 
 set -e -o pipefail
 
+# Run custom-built cipd client. This is useful when cipd is not available upstream
+# (i.e. unsupported platform).
+if [ ! -z "${CUSTOM_CIPD_CLIENT}" ]; then
+  exec "${CUSTOM_CIPD_CLIENT}" "${@}"
+fi
+
 # Export for other depot_tools scripts to re-use.
 export DEPOT_TOOLS_DIR="${DEPOT_TOOLS_DIR:-$(dirname "${BASH_SOURCE[0]}")}"
 export DEPOT_TOOLS_UNAME_S="${DEPOT_TOOLS_UNAME_S:-$(uname -s | tr '[:upper:]' '[:lower:]')}"
diff --git a/detect_host_arch.py b/detect_host_arch.py
index 317b12a79..500e5dd92 100755
--- a/detect_host_arch.py
+++ b/detect_host_arch.py
@@ -34,6 +34,8 @@ def HostArch():
         host_arch = 's390'
     elif host_arch.startswith('riscv'):
         host_arch = 'riscv64'
+    elif platform.system() == 'OS/390':
+        host_arch = 's390x'
 
     if host_arch == 'arm64':
         host_platform = platform.architecture()
diff --git a/download_from_google_storage.py b/download_from_google_storage.py
index f7e74e2bf..950a23999 100755
--- a/download_from_google_storage.py
+++ b/download_from_google_storage.py
@@ -35,6 +35,7 @@ PLATFORM_MAPPING = {
     'win32': 'win',
     'aix6': 'aix',
     'aix7': 'aix',
+    'zos': 'zos',
 }
 
 
diff --git a/gclient.py b/gclient.py
index 82c3909bc..9a4b16e10 100755
--- a/gclient.py
+++ b/gclient.py
@@ -1536,6 +1536,7 @@ _PLATFORM_MAPPING = {
     'linux': 'linux',
     'win32': 'win',
     'aix6': 'aix',
+    'zos': 'zos',
 }
 
 
@@ -1624,6 +1625,7 @@ class GClient(GitDependency):
         "ios": "ios",
         "fuchsia": "fuchsia",
         "chromeos": "chromeos",
+        "zos": "zos",
     }
 
     DEFAULT_CLIENT_FILE_TEXT = ("""\
diff --git a/gclient_utils.py b/gclient_utils.py
index eb911e7a0..dadd208e0 100644
--- a/gclient_utils.py
+++ b/gclient_utils.py
@@ -632,7 +632,7 @@ def CheckCallAndFilter(args,
         # subprocess when filtering its output. This makes the subproc believe
         # it was launched from a terminal, which will preserve ANSI color codes.
         os_type = GetOperatingSystem()
-        if sys.stdout.isatty() and os_type != 'win' and os_type != 'aix':
+        if sys.stdout.isatty() and os_type not in ['win', 'aix', 'zos']:
             pipe_reader, pipe_writer = os.openpty()
         else:
             pipe_reader, pipe_writer = os.pipe()
diff --git a/gsutil.py b/gsutil.py
index 353476068..ea0282705 100755
--- a/gsutil.py
+++ b/gsutil.py
@@ -37,7 +37,7 @@ LUCI_AUTH_SCOPES = [
 
 
 # Platforms unsupported by luci-auth.
-LUCI_AUTH_UNSUPPORTED_PLATFORMS = ['aix']
+LUCI_AUTH_UNSUPPORTED_PLATFORMS = ['aix', 'zos']
 
 
 class InvalidGsutilError(Exception):