diff --git a/changelog.d/19756.misc b/changelog.d/19756.misc
new file mode 100644
index 00000000000..2450505b531
--- /dev/null
+++ b/changelog.d/19756.misc
@@ -0,0 +1 @@
+Force keyword-only args for `Duration` (prevent footgun) so people have to specify which time unit they want to us.
diff --git a/synapse/util/duration.py b/synapse/util/duration.py
index 135b9808520..a1abe944b53 100644
--- a/synapse/util/duration.py
+++ b/synapse/util/duration.py
@@ -32,6 +32,33 @@ class Duration(timedelta):
     ```
     """
 
+    # Using `__new__` (instead of `__init__`) because that's what `timedelta` uses
+    def __new__(
+        cls,
+        # The whole goal of overriding `__new__` is to require keyword-only arguments.
+        # Without this, `Duration(5)` would create a duration represnting 5 *days*
+        # (timedelta's default), but callers almost certainly want to specify which unit
+        # like seconds or hours.
+        *,
+        days: float = 0,
+        seconds: float = 0,
+        microseconds: float = 0,
+        milliseconds: float = 0,
+        minutes: float = 0,
+        hours: float = 0,
+        weeks: float = 0,
+    ) -> "Duration":
+        return super().__new__(
+            cls,
+            days=days,
+            seconds=seconds,
+            microseconds=microseconds,
+            milliseconds=milliseconds,
+            minutes=minutes,
+            hours=hours,
+            weeks=weeks,
+        )
+
     def as_millis(self) -> int:
         """Returns the duration in milliseconds."""
         return int(self / _ONE_MILLISECOND)