Dauphin County is a county in the U.S. state of Pennsylvania. As of the 2010 census, the population was 268,100. The county seat and the largest city is Harrisburg, Pennsylvania's state capital and tenth largest city. The county was created/"erected" on March 4, 1785, from part of Lancaster County and was named after Louis-Joseph, Dauphin of France, the first son of Louis XVI.