Fabiano, who is currently sidelined for around six weeks with a knee injury suffered in last weekend's defeat to Athletic Bilbao, has agreed a four-year deal and will cost the Brazilian club €7.6 million.
Fabiano said: "Money is not everything in life. Nothing could pay for how happy I am to once again be able to wear the shirt of the team closest to my heart.
"I owe everything I have to Sao Paulo. I'm going to sweat for this shirt and I'm going to score many goals. I'm already thinking about coming out of the dressing room tunnel at the Morumbi stadium."