Fred, who started the match before making way with 15 minutes remaining in the first half for the former Brazil striker, scored the winner right in front of goal from Neymar's low left cross in the 22nd minute.
Ronaldo, 34, retired in February saying he struggled to keep his weight in check for the rigours of top flight soccer due to a thyroid problem.
The crowd at the Pacaembu were unhappy with Brazil's second-half performance in its last warm-up before defending the Copa America in Argentina next month, jeering the team towards the end and chanting Ronaldo's name.
Brazil, held 0-0 by the Netherlands in Goiania on the weekend, had a promising first half and managed to set up the chubby "Phenomenon", who played at walking pace, with three chances - but he missed them all.
The striker, who played for both Milan clubs and Spanish rivals Barcelona and Real Madrid in a brilliant career, was inside the six-yard box ready to tap in Maicon's low cross from the right but goalkeeper Ciprian Tatarusanu blocked.
Robinho then set the unmarked Ronaldo up with an easier chance but the holder of a record 15 World Cup goals and two winners medals sliced the ball way over the top.
Neymar then pulled the ball back from the goalline on the right and Ronaldo shot low, bringing Tatarusanu to a good diving save.
"People, you are too much. Excuse me, I had goal chances and I didn't manage to score in my last match, my last 15 minutes," said Ronaldo, wrapped in a Brazilian flag, after a lap of honour of the Pacaembu where he played for his last club Corinthians.
"I can only thank you for all your support, all the love the Brazilian people have given me all these years," he told the crowd from the centre circle at half-time.
Romania's best chance was a fierce free kick by Gabriel Muresan from 30 metres out that hit Victor's bar midway through the first half.
-Reuters
Tags: sport, football, brazil
