Rivaldo was given a hero's welcome by dozens of AC Milan fans as the World Cup sensation promised to end a streak of dismal seasons for the club.
AC Milan dominated Italian football in the late-80s and early '90s, but has had a comparatively dry spell recently.
"With the help of the teammates, I hope to win AC Milan the league title and the Champions Cup on my first season in Italy," Rivaldo told reporters.
Fans gathered in front of Rivaldo's hotel went wild when he appeared on the balcony, waving the red-and-black jersey of the Milan team.
AC Milan managing director Adriano Galliani said having Rivaldo on the squad was a long-held dream.
"We are glad to have signed one of the greatest, if not the greatest, player in the world," said Galliani.
He did not give details of Rivaldo's contract, but reports said the 30-year-old Brazilian forward, who helped Brazil win its fifth World Cup title this summer, would get £3million a year for three years, in addition to commercial contracts.