Ronaldo grew up in the poor Rio de Janeiro suburb of Bento Ribeiro. He began playing football as a junior for the neighbourhood Social Ramos Club at age 12 and two years later joined São Cristóvão in the Carioca League. By 1992 he was playing in the Brazilian championship for Cruzeiro. He was a member of Brazil's under-17 South American championship team in 1991 before joining the national side in 1994.

Ronaldo was transferred in 1994 from Cruzeiro to PSV Eindhoven of The Netherlands, where he scored 55 goals in 56 games and won the 1995 league championship and the 1996 Dutch Cup. After switching to F.C. Barcelona of Spain for the 1996–97 season, he scored 34 goals in 37 appearances and helped his team capture the Spanish Super Cup. Because of his success, Ronaldo was paid $27 million by F.C. Internazionale (Inter Milan) to play for the Italian club in 1997, a record at the time. At Inter his excellent dribbling skills and knack for scoring goals earned him the nickname “Il Fenomeno.” In 1997 Ronaldo became the first player to win FIFA's Player of the Year award two years in a row. In 1999, however, he suffered a serious knee injury that left him unable to play for almost two years.

Dismissing concerns that his career was over, Ronaldo returned to competitive play in 2001. At the 2002 World Cup he scored eight goals to earn the Golden Boot award as the tournament's top scorer and helped Brazil win its fifth World Cup championship. He then announced that he was leaving Inter for Real Madrid of Spain. After much wrangling, Real agreed to pay a transfer fee of about $46.3 million. In 2002 Ronaldo was named both FIFA Player of the Year and European Footballer of the Year (an award he also had received in 1997). At the 2006 World Cup he scored three goals to bring his career total at the tournament to a record-setting 15. While playing for the Italian powerhouse A.C. Milan in 2008, Ronaldo ruptured a tendon in his left knee—the same type of injury that occurred in his right knee in 1999—which some thought would put his career in jeopardy. In December 2008 a fully recovered Ronaldo signed with the Corinthians in São Paulo.