The 30-year-old has long been linked with a move away from Milan after failing to make an impression at San Siro in recent seasons.
Orlando City - who are to become an MLS franchise in 2015 - were said to have been in contention for his signature.
Yet Santos have ultimately won the race to sign Robinho as they bid to continue their decent start to the Brazilian Serie A campaign.
The move marks the second time that Robinho has joined Santos on loan since leaving for Real Madrid in 2005. He won the Paulista title and Copa do Brasil during his previous stint at the club in 2010.
"It's my home," Robinho said. "It's a place where I feel very good, where I showed up to the football world.
"I hope to repay all the affection of the fans on the pitch. I want to put Santos where it has to be, which is in the highest place in the table."
Robinho won the Brazilian Serie A twice with Santos after coming through their academy ranks, before making the move to Real.
He then added to his list of honours by winning two La Liga crowns with Real prior to an unsuccessful spell with Manchester City.
The Sao-Vicente born forward helped Milan to Scudetto glory in 2010-11, scoring 15 goals, but has netted just 17 times in three seasons since.
Santos sit sixth in Serie A after 13 games, and will hope Robinho can help them to victory against Corinthians on Sunday.
