San Martino in Rio (Reggiano: ) is a comune (municipality) in the Province of Reggio Emilia in the Italian region Emilia-Romagna, located about northwest of Bologna and about northeast of Reggio Emilia. Remarkable the church of San Carlo Borromeo.
San Martino in Rio borders the following municipalities: Campogalliano, Correggio, Reggio Emilia, Rubiera.