San Martino in Rio is a comune located in the Province of Reggio Emilia within the Italian region of Emilia-Romagna. It is situated approximately 50 kilometres northwest of Bologna and about 12 kilometres northeast of Reggio Emilia.
The municipality is notable for its church of San Carlo Borromeo. As of December 31, 2024, the population of San Martino in Rio was estimated to be 8,218 residents.