The 2008–09 Adelaide United FC season was the club's fourth A-League season since the inception in the Australian football league since it first started in 2005. The club competed in the 2008–09 A-League, 2008 AFC Champions League and 2008 FIFA Club World Cup.

For the 2008–09 season, Adelaide United made some significant signings to strengthen the squad, most notably striker Cristiano from Willem II for two years on a free transfer. Another key signing to fill the vacancy left by the retirement of Richie Alagich was Alemão formerly of CA Juventus. Saša Ognenovski joined the club from Queensland Roar to help shore up the defence, midfielder Paul Reid joined from Brighton & Hove Albion F.C., while promising youngsters Scott Jamieson, Daniel Mullen, Mark Birighitti and Robert Younis also signed. Bobby Petta, Shaun Ontong, Milan Susak, Dez Giraldi and Robert Bajic were all released, Bruce Djite was sold for an A-League record of A$850,000 to Genclerbirligi, whilst Nathan Burns also transferred to pursue his European dreams, joining Greek first division outfit AEK Athens for A$500,000. Kristian Sarkies, Lucas Pantelis and Fabian Barbiero all had their contracts renewed.