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."
"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."
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 Roberto Carlos has rescinded his contract with Corinthians.
