The 2009 American Le Mans Series season was the 39th season for the IMSA GT Championship, with it being the eleventh season with the American Le Mans Series moniker. It began on March 21, 2009, and ended on October 10, 2009 after ten events. The series was composed of Le Mans Prototypes (LMP) and Grand Tourer (GT) race cars divided into four classes: LMP1, LMP2, GT1, and GT2. A fifth category, known as the Challenge class, was also added for select races and featured Porsche 997 GT3 Cup cars from the IMSA GT3 Cup Challenge. 2009 was also the final year for GT1, with Corvette Racing abandoning the class after Long Beach and switching over to much more competitive GT2 (renamed GT) class.
This was the first time that the Michelin Green X Challenge championship was used throughout a full ALMS season after its initial introduction at the 2008 Petit Le Mans. Two champions were determined by the entry which is the most efficient over the season within the LMP and GT categories.
Patrón Highcroft Racing duo David Brabham and Scott Sharp won the main LMP1 standings, holding off De Ferran Motorsports pairing Gil de Ferran and Simon Pagenaud at the final round of the season at Mazda Raceway Laguna Seca. Adrian Fernández and Luis Díaz won the LMP2 title, in the final season of Fernández Racing, who closed at the end of the 2009 season. In GT2, Flying Lizard Motorsports' Patrick Long and Jörg Bergmeister won six races en route to the championship title. Husband-and-wife Martin and Melanie Snow won the inaugural GT Challenge class title, that ran at half of the season's ten races.