Kitsap County is a county in the U.S. state of Washington. As of the 2020 census, its population was 275,611. Its county seat is Port Orchard; its largest city is Bremerton. The county, formed out of King County and Jefferson County on January 16, 1857, is named for Chief Kitsap of the Suquamish Tribe. Originally named Slaughter County, it was soon renamed.