Catoctin Creek is a 27.9 mi tributary of the Potomac River in Frederick County, Maryland, US. Its source is formed north of Myersville. In Myersville, it merges with the Little Catoctin Creek at Doub's Meadow Park. From there, it flows directly south through Middletown, and the southern Middletown Valley for the entire length of the stream collecting several smaller tributaries. It makes a crossing with the Chesapeake and Ohio Canal at the Catoctin Aqueduct, one of just 11 of these structures to survive. Catoctin Creek enters the Potomac River east of Brunswick.