Butler, a borough and the county-seat of Butler county, Pennsylvania, U.S.A., on Conoquenessing Creek, about 30 m. N. of Pittsburg. Pop. (1890) 8734; (1900) 10,853, of whom 928 were foreign-born; (1910 census) 20,728. It is served by the Pennsylvania, the Baltimore & Ohio, the Buffalo, Rochester & Pittsburg, and the Bessemer & Lake Erie railways, and is connected with Pittsburg by two electric lines. It is built on a small hill about 1010 ft. above sea-level, and commands extensive views of the surrounding valley. The Butler County hospital (1899) is located here. A fair is held in Butler annually. Oil, natural gas, clay, coal and iron abound in the vicinity, and the borough has various manufactures, including lumber, railway cars (especially of steel), paint, silk, bricks, plate-glass, bottles and oil-well tools. The value of the city's factory products increased from $1,403,026 in 1900 to $6,832,007 in 1905, or 386.9%, this being much the greatest rate of increase shown by any city in the state having in 1900 a population of 8000 or more. Butler was selected as the site for the county-seat of the newly-formed county in 1802, was laid out in 1803, and was incorporated in the same year.

The county and the borough were named in honour of General Richard Butler, a soldier in the War of Independence and leader of the right wing of General St Clair's army, which was sent against the Indians in 1791 and on the 4th of November was defeated, Butler being killed in the engagement.