Ronaldo de Assis Moreira (born 21 March 1980), commonly known as Ronaldinho Gaúcho (Brazilian Portuguese: [ʁonawˈdʒĩɲu ɡaˈuʃu]) or simply Ronaldinho, is a Brazilian former professional footballer who played as an attacking midfielder or left winger. Widely regarded as one of the greatest players of all time, he won two FIFA World Player of the Year awards and a Ballon d'Or. He is the only player ever to have won a World Cup, a Copa América, a Confederations Cup, a Champions League, a Copa Libertadores and a Ballon d'Or. A global icon of the sport, Ronaldinho was renowned for his dribbling abilities, free-kick accuracy, his use of tricks, feints, no-look passes, and overhead kicks, as well as his ability to score and create goals. During his career, he was one of the most valuable footballers in the world. He is also known by the nickname "O Bruxo" ('The Wizard').

Ronaldinho made his career debut for Grêmio, in 1998. Aged 20, he moved to Paris Saint-Germain in France, where he won the UEFA Intertoto Cup, before signing for Barcelona in 2003. In his second season with Barcelona, he won his first FIFA World Player of the Year award as Barcelona won the 2004–05 La Liga title. The season that followed is considered one of the best in his career as he was integral in Barcelona winning the 2005–06 UEFA Champions League, their first in fourteen years, and another La Liga title, giving Ronaldinho his first career double, receiving the 2005 Ballon d'Or, and his second FIFA World Player of the Year in the process. After scoring two solo goals in the first 2005–06 El Clásico, Ronaldinho became the second Barcelona player, after Diego Maradona in 1983, to receive a standing ovation from Real Madrid fans at the Santiago Bernabéu. Due to these successes, Ronaldinho is widely credited with changing Barcelona's history.

Following a second-place La Liga finish to Real Madrid in the 2006–07 season and an injury-plagued 2007–08 season, Ronaldinho suffered a decline in his performances—due to a decrease in dedication and focus towards football—and departed Barcelona to join AC Milan, where he won the 2010–11 Serie A. He returned to Brazil to play for Flamengo in 2011 and Atlético Mineiro a year later where he won the 2013 Copa Libertadores, before moving to Mexico to play for Querétaro and then back to Brazil to play for Fluminense in 2015. Ronaldinho accumulated numerous other individual awards in his career: he was included in the UEFA Team of the Year and the FIFA World XI three times each, and was named UEFA Club Footballer of the Year for the 2005–06 season and South American Footballer of the Year in 2013; in 2004, he was named by Pelé in the FIFA 100 list of the world's greatest living players. In 2009, he was voted World Player of the Decade 2000s, ahead of Lionel Messi and Cristiano Ronaldo.

In his international career with Brazil, Ronaldinho earned 97 caps, scored 33 goals, and represented them in two FIFA World Cups. After debuting with the Seleção by winning the 1999 Copa América, he was an integral player in the 2002 FIFA World Cup winning team, positioned alongside Ronaldo and Rivaldo in an attacking trio, and was named in the FIFA World Cup All-Star Team. He captained his team to the 2005 FIFA Confederations Cup title and was named man of the match in the final. He also captained the Brazil Olympic team to a bronze medal in men's football at the 2008 Summer Olympics.

Ronaldo de Assis Moreira was born on 21 March 1980 in the city of Porto Alegre, the state capital of Rio Grande do Sul, Brazil. His mother, Miguelina Elói Assis dos Santos, was a salesperson who studied to become a nurse. His father, João de Assis Moreira, was a shipyard worker and a footballer for the local club Esporte Clube Cruzeiro (not to be confused with the larger Cruzeiro Esporte Clube). After Ronaldo's elder brother Roberto signed with Grêmio, the family moved to a home in the more affluent Guarujá section of Porto Alegre, which was a gift from Grêmio to convince Roberto to stay at the club. Still, Roberto's career was ultimately cut short by injury. When Ronaldo was eight years old, his father hit his head and drowned in the swimming pool at their new home. Roberto has acted as Ronaldo's manager, while his sister Deisi has worked as his press coordinator.

Ronaldo's football skills began to blossom at the age of eight, and he was first given the nickname Ronaldinho—inho, meaning 'small'—because he was often the youngest and the smallest player in youth club matches. He developed an interest in futsal and beach football, which later expanded to organized football. Many of his signature moves originate from futsal, especially his ball control. His first brush with the media came at the age of 13, when he scored all 23 goals in a 23–0 victory against a local team. Ronaldinho was identified as a rising star at the 1997 U-17 World Championship in Egypt, in which he scored two goals on penalty kicks.

"I've worked with some great players in my time and all at a very interesting period in their careers, nineteen to twenty years old. But, with due respect to the others, Ronaldinho was a cut above the rest."

Ronaldinho's career began with the Grêmio youth squad. He made his senior side debut during the 1998 Copa Libertadores. 1999 saw the emergence of the 18-year-old Ronaldinho, with 22 goals in 47 matches, and he put in headlining displays in derbies against Internacional, most notably on 20 June 1999 in the Rio Grande do Sul State Championship final. In a match-winning performance, Ronaldinho embarrassed Internacional's Brazilian legend and 1994 World Cup-winning captain Dunga, flicking the ball over his head on one occasion, and leaving him flat-footed in a mazy dribble on another. Ronaldinho achieved further success with Grêmio, winning the inaugural Copa Sul.