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