In 2014, during the archaeological investigations carried out by the University of Bologna (Department of History and Cultures – Section of Archaeology), within the Ostia Marina Project, in the suburban neighborhood out of Porta Marina (block IV, ix), a new building has been found with outstanding Mithraic features. The building, for the special type of the marble floor of the spelaeum, has been conventionally called the “Mithraeum of colored marbles”. The spelaeum has a single bench, a ritual well and a flowerbed for a sacred plant. It differs clearly both in form and size from the typical patterns of the mithraea discovered until now in ancient Ostia. On the basis of the currently available data, a very late chronology (end of 4th century AD) can be proposed for the building.