Barca have paid a total of 4.5 million euros to sign the 19-year-old forward.
ESPN reports Sao Paulo have confirmed the player's sale to the LaLiga Santander side.
Barcelona had bought a purchase option on the player for 1m euros at the beginning of the year and they had until July 15 to activate for an additional 3.5m euros.
The Spaniards will hold 70 percent of his rights, with Sao Paulo taking 30 percent of any future sale. Maia arrived at Sao Paulo aged 14 and has won seven titles across the club's different age levels.