Xaus's father gave him his first motorcycle — a Montesa 25cc — at the age of five. By fourteen he was competing on dirt bikes, before his father redirected him toward road racing. He contested the 125cc Catalan Championship, the 125cc Solo Moto Criterium, and won the 80cc Catalonia Supermoto Championship in the same year.
His progression through the junior categories was methodical. He raced in the Open Ducados Supersport series in Spain from 1994, improving from 17th to third place in his second year. In 1995 he entered the FIM Thunderbike trophy and contested four 250cc Grands Prix. He finished sixth in the Thunderbike series in 1996 before moving to World Supersport in 1997.
In 1999, Xaus claimed his first professional victory at Misano in the Supersport World Championship while finishing fifth overall. He built on that the following year with a factory Ducati Supersport ride, taking another win and finishing seventh.
Xaus joined the factory Ducati Superbike team for 2001 alongside reigning champion Troy Bayliss. His season started slowly, but in race two at Oschersleben he became the first Spaniard ever to win a race in the Superbike World Championship — a landmark result. Two second places at Assen, where he helped Bayliss clinch the championship, and a second victory at Imola gave him sixth overall for the season.
Sixth place again in 2002 was followed by the most successful campaign of his Superbike career in 2003. Xaus scored fifteen podiums and seven victories, eventually finishing runner-up to teammate Neil Hodgson in the championship. The year confirmed his status as one of the fastest Superbike riders of his generation.
Xaus moved to MotoGP in 2004 with the satellite D'Antin Ducati team. Severely underfunded, the team could not afford the testing time that would have helped a rider adapt to prototype machinery, but Xaus outperformed his circumstances and outpaced teammate Hodgson. Consistent points finishes and a first podium result at Qatar earned him the "Rookie of the Year" title and eleventh place overall — a creditable debut in the premier class.
The 2005 season with Fortuna Yamaha alongside friend Toni Elías proved far more difficult. The Yamaha's power delivery and chassis characteristics were poorly suited to Xaus's forceful riding style. He crashed frequently throughout the year, finishing 16th overall with a best result of tenth. After two seasons, he returned to Superbikes.
Xaus joined the Italian satellite Ducati team Sterilgarda Berik for 2006. His fast charges continued to end in crashes more often than results would have preferred, and he placed 14th overall, though he set the fastest lap twice during the season.
In 2007, Xaus produced his best return period since 2003. He finished sixth in the championship with 201 points and delivered the team's first ever race victory, at Valencia. It was a significant result for a rider many had assumed was past his competitive peak.
For 2008, teammate Max Biaggi joined Xaus at what became Sterilgarda-GoEleven. Xaus was second in race two at the opening round in Qatar and won at Misano, but the season also contained a notable controversy at Donington Park: believing he had finished third in a rain-stopped race, he was disqualified on the podium for not returning to the pits quickly enough after crashing before the red flag. His public response on the podium — refusing to leave and confronting marshals and the promoted third-place finisher, his own teammate Biaggi — drew widespread attention.
In June 2008, Xaus signed to race the BMW S1000RR for the factory BMW Motorrad team in the 2009 Superbike World Championship, becoming one of the early protagonists of BMW's return to the series.
In 2024, Xaus founded BRL Europe, a bagger motorcycle race series, and competed in it himself — demonstrating a continued attachment to competition long after his world championship career ended.
Rubén Xaus remains one of the most recognisable figures of the Superbike era of the early 2000s. His historic first win for a Spanish rider at Oschersleben in 2001, the 2003 runner-up campaign, and a MotoGP "Rookie of the Year" award collectively define a career that balanced genuine pace with an aggressive style that was simultaneously his greatest asset and his most consistent liability.