diff --git a/synapse/_scripts/synctl.py b/synapse/_scripts/synctl.py
index 1ab36949c741..b4c96ad7f3e2 100755
--- a/synapse/_scripts/synctl.py
+++ b/synapse/_scripts/synctl.py
@@ -24,7 +24,7 @@
 import subprocess
 import sys
 import time
-from typing import Iterable, Optional
+from typing import Iterable, NoReturn, Optional, TextIO
 
 import yaml
 
@@ -45,7 +45,7 @@
 --------------------------------------------------------------------------------"""
 
 
-def pid_running(pid):
+def pid_running(pid: int) -> bool:
     try:
         os.kill(pid, 0)
     except OSError as err:
@@ -68,7 +68,7 @@ def pid_running(pid):
     return True
 
 
-def write(message, colour=NORMAL, stream=sys.stdout):
+def write(message: str, colour: str = NORMAL, stream: TextIO = sys.stdout) -> None:
     # Lets check if we're writing to a TTY before colouring
     should_colour = False
     try:
@@ -84,7 +84,7 @@ def write(message, colour=NORMAL, stream=sys.stdout):
         stream.write(colour + message + NORMAL + "\n")
 
 
-def abort(message, colour=RED, stream=sys.stderr):
+def abort(message: str, colour: str = RED, stream: TextIO = sys.stderr) -> NoReturn:
     write(message, colour, stream)
     sys.exit(1)
 
@@ -166,7 +166,7 @@ def stop(pidfile: str, app: str) -> Optional[int]:
 )
 
 
-def main():
+def main() -> None:
 
     parser = argparse.ArgumentParser()