"I have dreamed about this for a long time, it's an unbelievable team and club and I'm very happy to be here," Cedric told Inter's TV channel. "I think my style of football is totally adapted to Inter."
The 27-year-old has won 33 caps for Portugal and was in the team which beat France 1-0 to win Euro 2016. He also played in three matches at the World Cup last year although has since lost his place in the side to Joao Cancelo.
Born in Germany to Portuguese parents, Cedric was raised at Sporting where he played until 2015 before joining Southampton.
(Writing by Brian Homewood; editing by Clare Fallon)