Armstrong County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 65,558. The county seat is Kittanning. The county was organized on March 12, 1800, from parts of Allegheny, Westmoreland and Lycoming Counties. It was named in honor of John Armstrong, who represented Pennsylvania in the Continental Congress and served as a major general during the Revolutionary War. The county is part of the Greater Pittsburgh region of the state.