Hallowing Run (also known as Hollowing Run) is a tributary of the Susquehanna River in Northumberland County, Pennsylvania, in the United States. It is approximately 6.1 miles (9.8 km) long and flows through Lower Augusta Township. The watershed of the stream has an area of 7.71 square miles (20.0 km2). The stream is not designated as an impaired waterbody. It is a stream of "local importance" according to History of Northumberland County, Pennsylvania, and is located between Little Mountain and the Shamokin Hills. The area in its vicinity was settled by the first half of the 1800s and several bridges have been constructed over the stream. Its watershed is designated as a Warmwater Fishery and a Migratory Fishery.