Ronaldinho is regarded as one of the greatest and most skilful players of his generation. Due to his ability to score and create goals, he was capable of playing in several offensive positions, on either wing or in a central position. Throughout his career, he was deployed as a forward, a winger, although he usually played as a classic number 10 in an attacking midfield role. Despite primarily being a creative player, he was an accurate finisher with either foot, both from inside and outside the penalty area, and was also a free-kick and penalty kick specialist. Throughout his career, Ronaldinho was praised in particular for his technical skills and creativity; due to his pace, acceleration, agility, balance, ball control, and dribbling ability, he was capable of beating players when undertaking individual runs, often using feints in one on one situations. Among his repertoire of moves, Ronaldinho was one of the best exponents of the "Elastico", a move he learned by watching videos of one of his idols, the 1970s Brazilian star Rivelino.

Ronaldo de Assis Moreira was born 21 March 1980 in the city of Porto Alegre, the state capital of Rio Grande do Sul, Brazil. His mother, Dona Miguelina Elói Assis dos Santos (daughter of Enviro Assis), is a former salesperson who studied to become a nurse. His father, João de Assis Moreira, was a shipyard worker and footballer for local club Esporte Clube Cruzeiro (not to be confused with the larger Cruzeiro Esporte Clube). He suffered a fatal heart attack in the family swimming pool when Ronaldo was eight. After Ronaldo's older 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, but Roberto's career was ultimately cut short by injury. Today, Roberto acts as his manager, while his sister Deisi works as his press coordinator. Ronaldo's football skills began to blossom at the age of eight, and he was first given the nickname "Ronaldinho" 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. 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.

Ronaldinho is one of few Brazilian players to have played at every international age level. In 1997, he was part of the first Brazilian team to win the FIFA U-17 World Championship, which was held in Egypt, in which his first goal was a penalty against Austria in the first group match, which Brazil won 7–0. Ronaldinho finished with two goals and was awarded the Bronze Ball award as Brazil scored a total of 21 goals while only conceding 2.

Ronaldinho made his senior debut for Grêmio, in 1998. At age 20, he moved to Paris Saint-Germain in France 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 La Liga. The season that followed is considered one of the best in his career as he was instrumental in Barcelona winning the UEFA Champions League, their first in fourteen years, as well as another La Liga title, giving Ronaldinho his first career double. After scoring two spectacular solo goals in 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. Ronaldinho also received his second FIFA World Player of the Year award, as well as the Ballon d'Or.

After falling short in 1999 and 2003, Ronaldinho was the captain of Brazil and led his team to its second ever Confederations Cup title in 2005. He converted a penalty kick in a 3–2 semi-final win against host Germany and was named Man of the Match in a 4–1 victory over archrival Argentina in the final on 29 June. Ronaldinho scored three goals in the tournament and is currently tied with Mexican forward Cuauhtémoc Blanco as the tournament's all-time scorer with nine goals.

In 2000, Ronaldinho participated in the Summer Olympics in Sydney, Australia, with the U23 team. Earlier that year, Ronaldinho led Brazil to win the Pre-Olympic Tournament, scoring nine goals in seven matches. In the Olympics, however, Brazil were eliminated in the quarter-finals by Cameroon, who later won the gold medal. Ronaldinho appeared four times and scored only one goal, which came in the quarter-final defeat by Cameroon.

Ronaldinho was also influential in the 2001–02 Coupe de la Ligue, helping PSG reach the semi-finals where they were eliminated by Bordeaux. In a Round of 16 match against Guingamp, Ronaldinho scored two second half goals in the game after having entered the match as a half-time substitute. Despite Ronaldinho's initial success with the club, the season was marred by controversy with Paris Saint-Germain manager Luis Fernández, claiming that the Brazilian was too focused on the Parisian nightlife rather than football, and complained that his holidays in Brazil never ended at the scheduled times.

Ronaldinho participated in his first World Cup in 2002, as part of a formidable offensive unit with Ronaldo and Rivaldo, dubbed the "Three Rs", who were also on the 1999 Copa América winning squad. The World Cup was held in Korea and Japan, and Ronaldinho appeared in five matches during the tournament and scored two goals, as well as contributing several important assists. His first goal came in the group stage match against China PR, which Brazil won 4–0.

Ronaldinho's next international tournament was the 2003 Confederations Cup, in which he went scoreless as Brazil were eliminated in the group stage. The following year, he was dropped from Brazil's 2004 Copa América squad, as coach Carlos Alberto Parreira decided to rest his stars and used a largely reserve squad.

Ronaldinho has featured in EA Sports' FIFA video game series, appearing on the cover of FIFA Football 2004, FIFA 06, FIFA 07, FIFA 08 and FIFA 09. At the beginning of his career Ronaldinho signed a lucrative 10-year deal with sportswear company Nike (wearing Nike Tiempo R10 boots designed for him). He has appeared in Nike commercials, including the 2002 "Secret Tournament" commercial (branded "Scopion KO") directed by Terry Gilliam. His 2005 Nike advertisement, where he is given a new pair of boots and then proceeds to juggle a football and appears to repeatedly volley it against the crossbar of a goal and recover it without the ball touching the ground, went viral on YouTube, becoming the site's first video to reach one million views.

Ronaldinho finished the season with a career-best 26 goals, including seven in the Champions League, and was chosen for the UEFA Team of the Year for the third consecutive time and was named the 2005–06 UEFA Club Footballer of the Year. He was named in the six man shortlist for the 2006 Laureus World Sportsman of the Year, and was selected in the FIFA World XI.

Ronaldinho has had endorsements with many companies, including Nike, Pepsi, Coca-Cola, EA Sports, Gatorade and Danone. One of the world's highest paid players, in 2006 he earned over $19 million from endorsements. Having endorsed Pepsi for much of his career and appeared in commercials with David Beckham, Thierry Henry and Lionel Messi, Ronaldinho signed a deal with Coca-Cola in 2011, however this was terminated in July 2012 after he was caught drinking Pepsi in a news conference.

A wax sculpture of Ronaldinho was unveiled at Madame Tussauds Hong Kong in December 2007. Ronaldinho has had an official role with UNICEF, the United Nations Children's Fund, since February 2006. In 2011, he was recruited by the Joint United Nations Programme on HIV/AIDS to promote awareness among young people of the disease and how to avoid it. In March 2015, Ronaldinho was the sixth most popular sportsperson on Facebook, behind Cristiano Ronaldo, Messi, Beckham, Neymar and Kaká, with 31 million Facebook fans.

On 7 July 2008, Ronaldinho was named in Brazil's 2008 Summer Olympics squad as one of the over-age players. Barcelona initially blocked the move because of his then-upcoming Champions League commitments with the club, but the decision was later nullified following Ronaldinho's transfer to Milan, who in turn permitted him to make the trip to Beijing, China. Ronaldinho captained the team, and he scored his only two goals in a 5–0 victory over New Zealand before Brazil were beaten by Argentina in the semi-final. Brazil finished with the bronze medal after defeating Belgium 3–0 in the bronze medal match.

Despite having returned to good form and being named as a member of the 30-man provisional squad that was submitted to FIFA on 11 May 2010, he was not named in coach Dunga's final squad of 23 for the Brazilian squad in South Africa for the 2010 World Cup despite his deep Desire to participate in the competition. Critics claimed that the exclusion of players such as Ronaldinho, Alex Andre Pato, Adriano and Ronaldo signaled a move away from the classic Brazilian attacking "Joga Bonito" style of play. At the tournament, Brazil was eliminated by the Netherlands in the quarter-final.

In September 2011, Ronaldinho made his return to the national team under coach Mano Menezes in a friendly against Ghana at Fulham's Craven Cottage, playing the full 90 minutes in a 1–0 win for Brazil. He then had solid performances in back to back friendlies against Argentina in the same month. In October, he performed well against Mexico in a friendly, scoring a free kick to equalize after Dani Alves was sent off. Brazil went on to win the match with a goal from Marcelo.

In 2012, two Brazilian entomologists named a new species of bee, from Brazil, Eulaema quadragintanovem, stating that "the specific epithet honors the Brazilian soccer player Ronaldo de Assis Moreira, famous worldwide as 'Ronaldinho' and in Brazil as ‘Ronaldinho Gaúcho’. 'Quadraginta novem' means forty-nine, the number of Ronaldinho's T-shirt at Clube Atlético Mineiro (CAM), his former team in Brazil. Ronaldinho chose the number 49 as an homage to his mother, born in 1949."

Ronaldinho's good form continued in 2013, and in January he was unexpectedly called up by coach Luiz Felipe Scolari for a friendly against England played on 6 February at Wembley Stadium as part of The Football Association (FA)'s 150th anniversary. Ronaldinho started in what was his 100th cap (including non-official matches), and had a chance to score from the penalty kick, but his shot was saved by Joe Hart. Brazil lost the match 1–2. He was again called up for the Seleção, being named captain of the national team for an international friendly with Chile on 24 April 2013. However, Ronaldinho was not selected for the national team for the 2013 Confederations Cup and he was also omitted from Scolari's 2014 World Cup finals squad.

After becoming a free agent, Ronaldinho was offered contracts from English Conference South club Basingstoke Town and newly formed Indian Super League franchise Chennai Titans through their co-owner Prashant Agarwal, but eventually signed a two-year contract with Mexican club Querétaro on 5 September 2014. Ronaldinho made his debut for Querétaro in a 1–0 loss to Tigres UANL where he missed a penalty kick. In his next match, however, against Guadalajara, he had a much better game, setting up Camilo Sanvezzo to score as well as scoring himself from a penalty kick in a 4–1 win. On 30 October 2014, he scored a free kick against Atlas during an away match at the Estadio Jalisco.

On 11 July 2015, Ronaldinho announced his return to Brazil and signed an 18-month contract with Fluminense, but on 28 September, Ronaldinho reached a mutual agreement with the club to terminate the deal. He made nine appearances during his two-month stint at the club, failing to impress and being heavily criticized by the fans. Fluminense sporting Director Mario Bittencourt stated, "Ronaldinho asked us for a meeting. He respectfully told us he didn't feel he was able to perform as good as he wanted and that it was a bad situation for him. He made a great gesture in saying he wasn't being the player he felt he could be right now. I'll never speak about whether or not he is retiring. That's not something you say about a player of his calibre. He was always spectacular, as player and person."

On 16 January 2018, Ronaldinho confirmed his retirement from football through his brother/agent: "He has stopped, it is ended. Let’s do something pretty big and nice after the Russia World Cup, probably in August." Such a celebration would take place three years after his last appearance for Fluminense.