The 29-year-old Portuguese player was delighted with the move.
He said: "It's such a great club and there is the chance to fight for the championship and win cups here.
"When you join a club like Rangers, you can't expect anything but to win things."
He added: "Everyone here wants to be champions again and to bring happiness to the fans." The former Porto player also feels his new club will benefit from his four-year experience of the British game.
"That will help," he said. "I know the game in England and the UK already so I'm already used to the British game."
Rangers will be hoping the capture of Mendes will calm fans who were angry with the sale of defender Carlos Cuellar to Aston Villa.