Bastareaud played for Creteil Rugby youth squads and then moved to Massy. He went through the junior academy there and played for the third division club SU Massy. He attracted national attention and after two seasons he moved to Stade Français. He was included in the 2009 Six Nations Championship squad. He impressed in the match against Wales, which France won 21–16, on 27 February 2009, which was also his Six Nations Championship debut. He later played against England and as a replacement against Italy in the same competition. Matthieu was involved in the 2010 Six Nations Championship and was a key figure for the French side. He scored two tries against Scotland at Murrayfield. He was rested for the majority of the game against Italy but was brought on for the last 15 minutes.
In July 2011 Bastareaud was finally allowed to join Toulon. He had previously declared his wish to leave and with Stade Francais suffering major financial difficulties, his wish was eventually granted. He signed a 3-year contract. In May 2013 he was named as man of the match as Toulon won the 2013 Heineken Cup Final by 16–15 against Clermont Auvergne. Despite the presence of French international captain Guilhem Guirado in Toulon, Bastareaud was selected as captain for the 2017/18 season, with South African team-mate Duane Vermeulen struggling with injury. After being caught swearing at opposition flanker Sebastian Negri in 2018, during a match against Benetton, Bastareaud was suspended for three weeks. It was announced in April 2019 that Bastareaud would leave Toulon after the 2018/19 season to join Rugby United New York in Major League Rugby.
International rugby
Bastareaud was included in the French tour to New Zealand and Australia in June 2009, but returned to France early after sustaining facial injuries. He initially claimed that he had been assaulted from behind by "four or five men" outside his Wellington hotel as he was returning from a night out. The New Zealand Police launched an investigation into the assault and turned up security camera footage of Bastareaud returning uninjured to his hotel with four other people at 5:22 am that night, confirming the assault could not have taken place. Bastareaud subsequently stated that he had been drunk and had sustained the injuries after tripping over a table in his hotel room, and that he had concocted the original story in order to avoid getting in trouble with team management. The incident proved to have diplomatic implications. Following the initial assault allegation, international media speculated on New Zealand's security and suitability as a tourist destination and host country, especially leading into the 2011 Rugby World Cup, spurring New Zealand Prime MinisterJohn Key to issue an official apology for the incident. Following the later revelations, French Prime MinisterFrançois Fillon sent a letter to Key, in which he apologised for the unfortunate affair and spoke of the strong relationship between France and New Zealand, a rare official intervention in sporting issues for the French. Then on 29 June, L'Equipe reported that he had been admitted to a psychiatric facility after attempting suicide. Pierre Camou, President of the French Rugby Federation has referred the case to their Disciplinary Committee and asked them to investigate. In 2010, he was selected in the French Barbarians squad to play Tonga on 26 November. Bastareaud started his first test in two years in 2017, against New Zealand in a 38–18 loss. Bastareaud had not made it onto the international field since he started in the 2015 Rugby World Cupquarter final, where the French lost to New Zealand 62–13. Bastareaud's two-year absence was cause by the fact that he had struggled with form loss and with injuries since the World Cup. Bastareaud missed the opening round of the 2018 Six Nations Championship due to suspension. He returned to international rugby on 23 February 2018 to start against Italy. Bastareaud had the final say in the fixture, scoring a try in the 73rd minute. His try was converted by Francois Trinh-Duc to make the final score against the Italians a 34–17 win, France's first international win in over 11 months. Bastareaud continued his return with good form and he ended up captaining France for the first time on 17 March 2018 during a 13–14 loss to Wales. On 19 June 2019, Bastareaud announced his retirement from international rugby after not being selected for France's Rugby World Cup warm-up matches.
