Kitsap County is located in the U.S. state of Washington. As of the 2010 census, its population was 251,133. Its county seat is Port Orchard, and its largest city is Bremerton. The county was formed out of King County, Washington, and Jefferson County, Washington on January 16, 1857 and is named for Chief Kitsap of the Suquamish tribe. Originally named Slaughter County, it was soon renamed.