Karen McDougal filed the lawsuit in Los Angeles Superior Court against American Media Inc, publisher of the National Enquirer, which paid her $150,000 in 2016 to keep quiet.
McDougal's lawsuit came on the same day that a New York state judge ruled Trump must face a defamation suit by California restaurateur Summer Zervos, who accused him of sexually harassing her after she appeared on his former reality TV show "The Apprentice."
New Yorker magazine last month reported on McDougal's alleged affair and a move by American Media to pay her for rights to her story, which it never published. American Media head David Pecker has described Trump as a "personal friend."
In the lawsuit, McDougal claimed she and Trump engaged in a 10-month extramarital "romantic relationship" in 2006 and 2007, a time period that overlaps with his alleged affair with adult film actress Stormy Daniels. Trump's wife Melania Trump gave birth to the president's youngest son Barron in 2006.
McDougal said her lawyer at the time, Keith Davidson, secretly negotiated with Michael Cohen, Trump's personal lawyer, on the AMI deal. Cohen has acknowledged arranging to pay Daniels for her silence during the 2016 presidential campaign.
She also called the agreement an illegal corporate donation from AMI to the Trump campaign that violated federal election law.
AMI said in a statement the company has a valid contract with McDougal and looks forward to reaching an amicable resolution with her. It said she has been free to respond to press inquiries about her relationship with Trump since 2016 and that the suggestion that AMI silenced her is without merit.
"AMI lied to me, made empty promises, and repeatedly intimidated and manipulated me. I just want the opportunity to set the record straight and move on with my life, free from this company, its executives, and its lawyers," McDougal, who was Playboy magazine's 1998 Playmate of the Year, said in a statement.
Davidson declined to comment, citing attorney-client privilege. Cohen did not immediately respond to a request for comment.
Daniels, whose real name is Stephanie Clifford, sued the president on March 6, stating Trump never signed an agreement for her to keep her quiet about an "intimate" relationship between them. Daniels received $130,000 under that agreement.
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