"I can't take any more," the 34-year-old star told the Estado de Sao Paulo newspaper.
"I wanted to continue, but I can't do it any more. I think of an action, but I can't do it the way I want to. It's time.
"It's the body that's hurting me," he told Globo television. "The head wants to go on, but the body can't take any more."
Ronaldo had already spoken in terms of retiring at the end of the year.
But a succession of injuries and the elimination of his club, Corinthians, from the Copa Libertadores, South America's most prestigious club tournament, appears to have pushed the date forward.