Ronaldo, 32, will join one of Brazil's biggest clubs, who have just won promotion back to the top flight after one season in Serie B, for the 2009 season as he attempts to re-ignite his career.
"I'm proud of myself," he said in an interview with Brazilian television. "This was never an easy fight at any moment and I'm winning.
"Corinthians believed in me, they have a fantastic project for 2009 and I'm very confident it will be a great year, although I know I will have a lot of difficulties at the start."
Ronaldo 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.
In April he was involved in an incident with three transvestites in Rio de Janeiro when he went with them to a motel with them in the belief they were call girls.
By October Ronaldo said he had put on so much weight following his injury that he was running out of clothes and last month he told Italy's Gazzetta dello Sport he doubted whether he would play again.
"If I didn't believe in myself, I wouldn't have made this sacrifice," said Ronaldo, referring to his rehabilitation. "I know it will be difficult to re-adapt but I like football and I believe I still have a lot to offer the game."
Ronaldo had been widely expected to sign for Flamengo, where he had been training for the last four months, and his move to Corinthians upset supporters of the Rio de Janeiro-based club, who burned photos of him after hearing the news.
"I understand perfectly," said Ronaldo. "I spent four months in Rio training at Flamengo but nothing happened, I didn't receive a project, any ideas, any offer. Corinthians will be an excellent challenge for me."
