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