The 30-year-old forward had been linked with a move to Major League Soccer, but instead heads back to the club where he began his career before moving to Real Madrid in 2005. He also had a spell there on loan from Manchester City in 2010 before joining Milan.
"This is my home," Robinho said on the Santos website. "It's a place where I feel very good and where I came into the world of football. I hope to repay the affection of all the fans on the pitch. I want to put Santos back where they should be, at the very top of the table."
Robinho had an injury-hampered season in Milan last year as the Rossoneri stumbled to an eighth-placed finish.
The 92-cap Brazil international is under contract with the Italian giants until June 2016 but had expressed his wish to leave the San Siro outfit this summer after four years at the club.
He had been expected to join compatriot and former Milan team-mate Kaka at new MLS franchise Orlando, but Santos were keen to secure a fan favourite.
"When we started the negotiations we knew the importance of the player both for the club and for the fans," said Santos sporting director Andre Zanotta. "It was a difficult negotiation but with a happy ending. Now we are hoping the documentation is done in time for the match against Corinthians on Sunday."