is a city in Butler County, Pennsylvania, United States located 35 miles north of Pittsburgh. The population was 15,121 at the 2000 census. It is the county seat of Butler County.
The city was named for Maj. Gen. Richard Butler, who fell at the Battle of the Wabash, also known as St. Clair's Defeat, in western Ohio in 1791. The first settlers were of Irish or Scottish descent and were driving westward from Connecticut. In 1802 the German immigrants began arriving, with Detmar Basse settling in Jackson Township in 1802 and founding Zelienople the following year. After George Rapp arrived in 1805 and founded Harmony, larger numbers of settlers followed. John A. Roebling settled Saxonburg in 1832, by which time most of the county was filled with German settlers.