"The Huffington Post is nothing without the bloggers who created the content," said Jonathan Tasini, a one-time Huffington Post blogger who filed the suit and is seeking class-action status on behalf of the bloggers.
Part opinion and part news, the left-of-centre website has to some extent relied on free contributions by celebrities, politicians and experts to drive traffic, turning it into a major online force since it started in 2005.
A spokesman for the website said he had not reviewed the lawsuit, but called the allegations "completely baseless".
"Our bloggers utilise our platform to connect and ensure that their ideas and views are seen by as many people as possible," spokesman Mario Ruiz said.
"It's the same reason hundreds of people go on TV shows to broadcast their views to as wide an audience as possible."
To what extent the website's sale value or estimated revenue was based on unpaid blogs is unclear, the lawsuit acknowledged, but it claimed the roughly 9000 unpaid bloggers should receive their fair cut.
The lawsuit also called for the website to release detailed information on the internet traffic to and from the blogs.
John Coffee Jr, a professor at New York's Columbia University Law School, said the lawsuit would likely be dismissed by a judge as the bloggers' decision to contribute to the website was a rational one, and that the internet site was within its rights to profit from the free content.