Adelphi is a village in Ross County, Ohio, United States. The population was 322 at the 2020 census.

Ross County is a county in the Appalachian region of the U.S. state of Ohio. As of the 2020 United States Census, the population was 77,093. Its county seat is Chillicothe, the first and third capital of Ohio. Established on August 20, 1798, the county is named for Federalist Senator James Ross of Pennsylvania. Ross County comprises the Chillicothe, OH Micropolitan Statistical Area, which is also included in the Columbus-Marion-Zanesville, OH Combined Statistical Area.