They allege that she violated the agreement by misrepresenting and disclosing information about the show to a number of people including freelance writer Peter Lance, who last year published a book about the series.
On Tuesday SEG Inc filed a countersuit against Stillman, a San Francisco lawyer who says the shows producer convinced two fellow contestants to vote her off to protect another contestant.
According to SEG, Stillman has violated a confidentiality contract, with a $5m (£3.4m) penalty charge attached, that she signed before appearing in the show.
They allege that she violated the agreement by misrepresenting and disclosing information about the show to a number of people including freelance writer Peter Lance, who last year published a book about the series.
Denials and damages
The CBS network had already denied Stillman's allegations that the show deceived the public by claiming it was a fair contest.
Survivor, based on a British format devised in the 1980s, pitted two "tribes" against each other on an island off the coast of Borneo. Each week, one of the tribes voted off a member.
Ms Stillman was the third contestant to be evicted from the island.
She is claiming damages for alleged fraud, breach of contract and unlawful business practises.
Fellow contestants Dirk Been and Sean Kenniff are also accused of conspiracy.
Ms Stillman also asked for compensation to cover lost prize money, plus $75,000 (£51,540) in damages.
Rudy Boesch, the contestant Stillman claims was favoured by the show's producer, made it to the final four contestants on Survivor, but the competition was won by corporate trainer Richard Hatch, who received $1m (£687,380).
The series was the most successful summer prime-time show in US TV history, with almost 60 million viewers tuning into the show's final episode.
A British version is being developed for ITV by production company Planet 24, under the control of Nigel Lythgoe, the LWT executive behind the success of Popstars.