St. Martin is an unincorporated community and former village in Brown County, Ohio, United States. The community is served by the Fayetteville (45118) post office. The population was 200 at the 2020 census. In November 2011, voters chose to unincorporate St. Martin, and it now falls solely under the jurisdiction of Perry Township.

Brown County is a county in the U.S. state of Ohio. As of the 2020 United States Census, the population was 43,676. The county seat is Georgetown. The county was created in 1818 and is named for Major General Jacob Brown, an officer in the War of 1812 who was wounded at the Battle of Lundy's Lane. Brown County is part of the Cincinnati-Middletown, OH-KY-IN Metropolitan Statistical Area.