The vehicle, which will use Mazda's next-generation rear-wheel-drive technology, is to be sold under the Mazda and Alfa Romeo brands, the latter owned by Fiat, with some differences including an engine unique to each.
Both are to be made in the southern Japanese city of Hiroshima, with production on the Alfa Romeo brand car scheduled to begin by 2015, they added.
"Establishing technology and product development alliances is one of Mazda's corporate objectives and this announcement with Fiat is an important first step in that direction," said Mazda President Takashi Yamanouchi.
Automakers have been striking technical and financial tie-ups in recent years amid intensifying global competition and soaring development costs.
Copyright Agence France-Presse, 2012