Kenai Peninsula Borough is a borough of the U.S. state of Alaska. As of the 2020 census, the population was 58,799, up from 55,400 in 2010. The borough seat is Soldotna, the largest city is Kenai, and the most populated community is the census-designated place of Kalifornsky. The borough includes most of the Kenai Peninsula and a large area of the mainland of Alaska on the opposite side of Cook Inlet.