Tenmile Run (also known as Ten Mile Run or Ten-mile Run) is a tributary of the Susquehanna River in eastern Columbia County, Pennsylvania, in the United States. It is 6.1 miles (9.8 km) long and is entirely in Mifflin Township. The stream's watershed has an area of 8.24 square miles. The first settlers in the area arrived in the late 1700s and a number of mills have been built on the stream. It is designated as a High-Quality Coldwater Fishery and as Class A Wild Trout Waters.