Needing just a draw to lift their 32nd championship trophy, Benfica started strongly in front of their own fans and were ahead within three minutes when when Cardozo pounced on a loose ball to volley in.
He could have added another moments later but arrived just too late to convert a David Luiz cross.
Rio Ave's Ricardo Chaves silenced a capacity crowd at the Luz Stadium with a headed equaliser on 70 minutes but Cardozo lifted spirits again eight minutes later, pouncing on another loose ball after a corner by Pablo Aimar.
The goal also earned Cardozo the top scorer's crown with 26 goals, one more than Porto's Colombian striker Radamel Falcao.
Their second-place finish, five points adrift of Benfica on 71 points, was their best ever league performance and earned them a place in next season's Champions League third qualifying round.
Last season's champions Porto finished third and will enter the Europa League next season together with fourth-placed Sporting and fifth-placed Maritimo.
