Jonathan Tasini accuses the site of not paying freelance bloggers while benefiting financially from the content they create.
Tasini's lawsuit is seeking class-action status on behalf of more than 9,000 writers and other content providers and asks for at least $105 million in damages.
AOL bought the Huffington Post for $315 million last month, which Tasini says prompted the lawsuit.
The suit names Huffington Post co-founders Arianna Huffington and Kenneth Lerer as defendants.
AOL says the lawsuit is without merit.