Armstrong County is a county located in the U.S. state of Pennsylvania. As of the 2010 census, the population was 68,941. 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.