Six models brought the claim that 13 agencies based in New York, including giants such as Ford, Elite and Wilhemina, worked together to keep prices down and commissions up.
Judge Harold Baer's ruling that they had "established the existence of an aggrieved class" paves the way for thousands more to join the anti-trust action.
At present supermodels negotiate their own contracts with the agencies, but the agencies charge almost all their other models 20% commission on what they earn. They pick up another 20% commission from those that hire them.
The models say that the 20% is illegal because New York law limit employment agency commissions to 10%.
Their case argues that the agencies have been working together as a cartel for about 30 years.
"Models who threaten to complain are told they will be blackballed," their case claims.
Carolyn Fears, one of the plaintiffs, told the Los Angeles Times: "When I was first starting, I went to the agency to get my paycheck and they were taking $1,500 [£950] out of it,"
"I said, 'What is this?'
One of the owners was standing behind me and said, 'That's to put you in the agency's book.
"I don't see you on the cover of Vogue. If you don't like it, get out.'"
Ms Fears joined the Ford agency and worked in New York and Europe, appearing on magazine covers and on advertising hoardings.
She said the altercation on her first payday left her in tears.
"It's intimidating, and you just want to please everybody," she said. "You're not thinking about commissions and fees."
"Who wants to bite the hand that feeds them?"
According to the case, the agencies in question represent more than half the New York market, in terms of the total volume of bookings.
In a deposition, the chief executive of the Ford agency argued that the models affected by a class action could "number thousands."
The agencies argued that each model's case must be treated individually - and each case tried separately - because each model was unique, with different looks, talent, gender and age.
They said that they were exempt from the 10% commission limit because they were managers rather than employment firms.
They also denied colluding to set rates that would prevent models shopping around for the best offer.
But the judge said a sample of contracts showed that "an overwhelming number of models were charged the same rate" - the standard 20% fee applied to about 95% of them.
"Here, plaintiffs have alleged in their complaint that defendants conspired to evade state pricing regulation laws by repackaging themselves as model management companies, when in reality they remained in substance employment agencies, still subject to state laws," he said.
The six models are all from California. They are in their thirties and forties and coming to the end of their careers.
"You start modelling so young that you are just so happy to be doing it," Ms Fears said.
"The majority of us make a normal, average living. There were years when I was borrowing money from my parents to get by.
"You naturally get wiser as you get older, and by that point you might be getting out of the industry and not care."
A lawyer for the Elite agency told the New York Daily News that the judge's ruling was "disappointing but not terribly surprising".
"The fact is we remain very confident that there is no basis for this substantive claim," he said. "It's a lousy claim."