Gillespie Run is a long 3rd order tributary to the Youghiogheny River in Allegheny County, Pennsylvania.
Gillespie Run rises about 2 miles west-southwest of Mustard, Pennsylvania, and then flows northeasterly to join the Youghiogheny River across from Sutersville.
Gillespie Run drains of area, receives about 39.5 in/year of precipitation, has a wetness index of 318.16, and is about 62% forested.