JOIN USThe tearful Corinthians striker announced his decision at a press conference in Sao Paulo.
"I'm stopping my career as a professional footballer," he said, his voice breaking. "It's been a beautiful, emotional, marvellous career.
"These last two years, I've had a long series of injuries, from one side to the other, one leg to the other, one muscle to the other. The pain pushed me to think about the end of my career."
Ronaldo also revealed that he suffered from a thyroid problem that had made it difficult to control his weight.
"Four years ago, in Milan, I discovered that I suffered from an under-active thyroid that slowed down my metabolism and that to control it, I had to take hormones that weren't authorised in football as they were considered a form of doping," he said.
"Lots of people must regret having made jokes about my weight. But I feel no anger towards anyone." Having earlier indicated that he hoped to play on until the end of the year, injuries and Corinthians' early elimination from the Copa Libertadores prompted him to bring forward his retirement.
During a glittering 14-year spell in Europe, Ronaldo scored goals at a prolific rate for PSV Eindhoven, Barcelona, Inter Milan, Real Madrid and AC Milan, before returning to Brazil in 2009.
He won the World Cup with Brazil in 2002 and became the leading scorer in World Cup finals tournaments when he scored his 15th goal at the 2006 tournament in Germany. He was twice named European Footballer of the Year, in 1997 and 2002, and finished his international career with 62 goals in 97 appearances.
Widely regarded as one of the greatest players of all time, 'El Fenomeno' (The Phenomenon) won the World Cup with Brazil for the first time in 1994, though on that occasion the 17-year-old was part of the squad and did not play.
In 1998, he was among the losing finalists, beaten 3-0 at Stade de France by a rampant French side that included the inspirational Zinedine Zidane. Brazil's star striker suffered a convulsive fit on the eve of the match and was removed from the starting line-up.
In a dramatic changing-room re-shuffle, he was reinstated just before kick-off but was a shadow of the intimidating forward who had scored four goals in the run-up to the final.
But the 2002 tournament was where he made history.
Shrugging off injury in the tournament, which was hosted by Japan and South Korea, he played a key role in helping Brazil to the trophy, scoring both goals in his country's 2-0 win over Germany in the final in Yokohoma.
At the 2006 World Cup in Germany he took his World Cup tally to 15 goals, surpassing the previous record held by Gerd Mueller by one goal. Ronaldo, who began his career with Cruzeiro, had to fight back from three career-threatening knee injuries over the course of his career.
He was sidelined while playing for Inter by a knee injury in 1999 only to damage the same knee in his comeback for the club in February 2000, effectively keeping him out of action until March 2002.
In February 2008 he ruptured a tendon in his right knee playing for AC Milan against Livorno, an injury that ended his career in Europe.