Larimer County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 359,066. The county seat and most populous city is Fort Collins. The county was named for William Larimer, Jr., the founder of Denver. Larimer County comprises the Fort Collins, CO Metropolitan Statistical Area. The county is located at the northern end of the Front Range, at the edge of the Colorado Eastern Plains along the border with Wyoming.