The former AC Milan midfielder has been unveiled at the club where he began his career, where he has now begun a loan spell from MLS side Orlando City
The 32-year-old had his contract with the Rossoneri terminated through mutual consent in June, before joining the Florida franchise.
He will play on loan at Sao Paulo, however, until Orlando join MLS for their inaugural season in 2015.
And the midfielder received a rapturous welcome from supporters at his unveiling at Morumbi on Sunday, with over 20,000 fans attending.
"It's an emotional moment, 11 years later, to be back at the club where I've grown up, where I started in football, to see all the supporters here, my friends and family," said Kaka.
"It's a very special day for me. Sao Paulo rocketed me into world football, and to be back is a memorable and very special day.
"I'm very happy, as my family and children are here too. I can't wait to play. I'm looking forward to playing alongside these two idols here [goalkeeper Rogerio Ceni and striker Luis Fabiano, both attending the presentation].
"Thank you all, I'm sure this will be very special. I don't know when I'll be back at playing, but I'll try to be back as soon I can."
The former Brazil international also talked about compatriot Neymar's injury, that will prevent him from playing in the rest of the World Cup for the Selecao, and echoed the sentiment of sadness being expressed throughout the host nation.
"As a lover of football, it's sad news," he added. "Both Brazil and the World Cup lose with his injury. This is a negative aspect for Brazil considering a technical approach, but I hope the team can take this as a motivation for this final rounds of the World Cup.
"What Neymar is going through is something unique... He's going through a hard situation, being out of a World Cup in his own country. We are now crying for him, but we'll have reasons to smile in the future."