Trumbull County is a county in the far northeast portion of the U.S. state of Ohio. As of the 2020 census, the population was 201,977. Its county seat and largest city is Warren, which developed industry along the Mahoning River. Trumbull County is part of the Youngstown–Warren, OH Metropolitan Statistical Area.

Warren is a city in and the county seat of Trumbull County, Ohio, United States. Located along the Mahoning River, Warren lies approximately 14 miles (23 km) northwest of Youngstown and 56 miles (90 km) southeast of Cleveland. The population was 39,201 at the 2020 census. The historical county seat of the Connecticut Western Reserve, it is the third largest municipality in the Youngstown–Warren metropolitan area after Youngstown and Boardman, and anchors the northern part of that area.