Willoughby Run
General data
- Name: Willoughby Run
- Water system: Potomac River
- Water type: Stream
- Progression: Marsh Creek (Monocacy tributary) -> Monocacy River -> Potomac River -> Chesapeake Bay -> Atlantic Ocean -> Planet Earth
- Climates: Temperate
- Continents: North America
- Countries: United States of America
Willoughby Run is a tributary of Marsh Creek in Adams County, Pennsylvania in the United States.