Alguersuari took over the role of reserve driver for the Red Bull Racing and Scuderia Toro Rosso Formula One teams from fellow Red Bull Junior driver Brendon Hartley in the second half of the 2009 Formula One season. Less than two weeks later, race driver Sébastien Bourdais left the Toro Rosso team after the 2009 German Grand Prix and Alguersuari was immediately expected to be his successor, despite the lack of an official confirmation. Four days later, Toro Rosso announced that Alguersuari would drive for the team at the 2009 Hungarian Grand Prix. He became the youngest ever Formula One driver at the age of 19 years and 125 days, breaking the record previously held by Mike Thackwell. He became only the seventh teenager to start a Grand Prix. Qualifying in last place following a mechanical problem, he finished the race in fifteenth, one place ahead of his teammate Sébastien Buemi. Through the rest of the season, he had little success, with his best result coming at the 2009 Brazilian Grand Prix, where he managed 14th place. At the remaining 8 races, he only finished 3 of them and retired in the other 5. During the season he also had a huge crash in the while he was attempting to chase for his first points.