Ashtabula County is the northeasternmost county in the U.S. state of Ohio. As of the 2020 census, the population was 97,574. The county seat is Jefferson, while its largest city is Ashtabula. The county was created in 1808 and later organized in 1811. The name Ashtabula derives from the Lenape language phrase ashte-pihële, which translates to 'always enough (fish) to go around, to be given away' and is a contraction of apchi ('always') + tepi ('enough') + hële. Ashtabula County is part of the Cleveland, OH Metropolitan Statistical Area.

Jefferson is a village in and the county seat of Ashtabula County, Ohio, United States. The population was 3,226 at the 2020 census. It is part of the Ashtabula micropolitan area, 50 miles (80 km) northeast of Cleveland.