Sao Caetano had taken a 22nd minute lead with a Marcelo Batatais header before Andre Santos rifled in the equaliser with a long-range shot in the 35th minute.
Ronaldo, playing his third game for Corinthians and starting for the first time in his home debut, swept the ball home with a first-time shot from Dentinho's low cross in the 51st minute.
The former Barcelona, PSV Eindhoven, AC Milan and Inter Milan player, who scored an injury-time equaliser in a 1-1 draw with arch-rivals Palmeiras on Sunday, went off in the 78th minute to a standing ovation from the 30,000 crowd at the Pacaembu.
Ronaldo, 32, ruptured a tendon in his left knee in a Serie A game playing for AC Milan against Livorno in February and later left the Italian club. He joined Corinthians in December in a surprise move and made his debut one week ago.
Corinthians stayed second in the Paulista championship and cut Palmeiras's lead to a point after Vandelei Luxemburgo's team could manage only a 1-1 draw with Ituano.
The Paulista is the largest of the regional championships which kick off the Brazilian season.
Former Brazil and South Africa coach Carlos Alberto Parreira made a winning start with Fluminense as they beat Volta Redonda 2-1.
Parreira quit South Africa last year for family reasons when he had been due to lead them at the 2010 World Cup, which they will host. He joined Fluminense on Friday after former Jamaica coach Rene Simoes was sacked by the club.
Thiago Neves gave Fluminense a 20th minute lead with a free kick but former Brazil defender Junior Baiano equalised six minutes later. Dario Conca scored the winner in the 58th minute.
