Williamsport is a town in Washington Township, Warren County, Indiana, United States. The population was 1,898 at the 2010 census. It is the county seat of Warren County and is the largest of the four incorporated towns in the county. Williamsport Falls, the highest waterfall in the state, is located in downtown Williamsport.
Williamsport was platted by (and named for) William Harrison on December 11, 1828, on the south end of the eastern fractional part of the northeast quarter of section 11, township 21, range 8. Harrison was the first to occupy the town in the fall of 1828; he built a log house at the east end of Main Street near the river and operated a ferry crossing there for several years. Though located on the opposite side of the river from the Wabash and Erie Canal, Williamsport sought to draw shipping traffic by constructing a short cut-off canal. Finished around 1852, the project led to the nickname "Side-Cut City".
Williamsport became the county seat in June 1829, taking the title away from short-lived Warrenton. Its post office was the first in the county and has operated since September 28, 1829. The town was incorporated in March 1854, at which time its population was 552.