The 34-year-old three-times FIFA World Player of the Year, a twice World Cup winner, said injuries and fitness problems prevented him from playing to his best form.
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"I came here to say today that I'm ending my career as a professional player," Ronaldo, with his two sons at his side, told a packed news conference at Corinthians' training complex in Sao Paulo.
"Everyone knows about my history of injuries...and those pains made me bring forward the end of my career."
Ronaldo, who had a contract with Corinthians until the end of the year, said his fitness problems were the overriding reason for his decision.
He said he discovered while playing for AC Milan in Italy that he was suffering from a thyroid problem.
"Four years ago at Milan I discovered I was suffering from a complaint called hypothyroidism, which slows down your metabolism and to control it I would have to take some hormones that are not permitted in football because of anti-doping," Ronaldo said.
"I wanted to clarify this on the last day of my career," added Ronaldo, who has struggled to keep his weight down and often suffered the jibe of "fatty" during his career.
"It's very hard to leave something that makes you so happy, to which you gave so much, which you love so much and could even continue doing, because mentally I still want to, but one also has to accept some defeats. I lost to my body.
"This is the time, I'll do other things in my life, I'll be tied to Corinthians for ever."
LIBERTADORES FAILURE
Ronaldo broke down in tears when he apologised to the Corinthians fans for failing to deliver the major prize of the Libertadores Cup and said he would still go to their matches as a supporter.
He had been signed with the Libertadores Cup in mind but Corinthians were dumped out of South America's elite club competition, which they have never won, in the preliminary round two weeks ago. They went out in the round of 16 in 2010, their centenary year.
This season's failure led to violent protests by fans with Ronaldo and his former Real Madrid and Brazil team mate Roberto Carlos the main targets. Roberto Carlos has rescinded his contract with Corinthians.
During a brilliant career riddled with goals for four major European clubs - Barcelona, Inter Milan, Real Madrid and AC Milan in that order - Ronaldo was voted FIFA world player of the year three times and also won the European footballer of the year award twice.
He won two World Cup winners medals with Brazil. He was an unused squad member when Brazil won the title in
