The former Manchester City and Real Madrid forward has agreed a deal with the Brazilian top-flight club for the remainder of the campaign.
Robinho had been without a team since leaving Turkish club Istanbul Basaksehir in August and the 36-year-old was delighted to return to familiar surroundings.
"This has always been my home," he said of Santos, who are currently seventh in the table after 14 games.
"My objective is to help on and off the pitch and get Santos in as high a place as possible."
Robinho started his career with Santos prior to moving to Europe, when he initially joined Real Madrid before becoming the first major signing for Manchester City after their takeover in 2008.
He twice returned to Santos on loan in 2010 and 2014, either side of a four-season spell with Milan.
As well as stints in China and Turkey, Robinho previously returned to Brazil to play for Atletico Mineiro in 2016 and 2017.
Robinho, who has won two league titles with Santos, made 100 appearances for Brazil between 2003 and 2017, scoring 28 times.
His return to Santos was billed as 'The Last Pedal' on social media, a twist on NBA legend Michael Jordan's 'Last Dance' with the Chicago Bulls.