The 2009–10 Adelaide United FC season was the clubs fifth A-League season. It includes the A-League 2009-10 season as well as any other competitions of the 2009-10 football (soccer) season.

After a stellar season in the A-League and internationally, Adelaide United began its pre-season without a financial owner; Nick Bianco relinquished his A-League licence back to Football Federation Australia (FFA). Regardless of this situation, the FFA ensured that Adelaide would have the funds available to recruit in the off-season and to maintain the club, whilst they would negotiate the new ownership deals. Adelaide United managed to secure major sponsors Modern Solar as well as Jim's Plumbing for their 2009–10 season.

At the end of the 2008-09 regular season Adelaide United released veteran Angelo Costanzo as well as Jonas Salley, Isyan Erdogan and Jason Spagnuolo, with Diego Walsh moving to New Zealand club Wellington Phoenix !Wellington Phoenix and Saša Ognenovski moving to K-League side Seongnam Ilhwa. Veteran Adelaide players Michael Valkanis, Daniel Beltrame and Paul Agostino retired at the end of the 2008-09 season.

Adelaide made a number of signings to replace the players which left in the off-season, most notably former Sydney FC defenders, Iain Fyfe and Mark Rudan, Korean youngster Inseob Shin and former Drogheda United midfielder Adam Hughes. Arguably the biggest off-season signing was former Cheltenham Town and Brentford striker, Lloyd Owusu, a capped Ghanaian international. The Reds also elevated a handful of players from their Youth squad to the senior team, namely Francesco Monterosso, Michael Marrone and Joe Costa.

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