"Defendant breached his fiduciary duties owed to Plaintiff by virtue of their attorney-client relationship by both revealing Plaintiff's confidences, and spreading falsehoods about Plaintiff, likely to be embarrassing or detrimental, and partook in other misconduct," the lawsuit said.
The lawsuit, filed in Miami federal court, accused Cohen of "an onslaught of fiduciary and contractual breaches" through numerous "inflammatory and false statements" about Trump in books, his podcast and media appearances.
"Defendant appears to have become emboldened and repeatedly continues to make wrongful and false statements about Plaintiff through various platforms. Such continuous and escalating improper conduct by Defendant has reached a proverbial crescendo and has left Plaintiff with no alternative but to seek legal redress through this action," the lawsuit said.
"Defendant chose to capitalize on his confidential relationship with Plaintiff to pursue financial gain and repair a reputation shattered by his repeated misrepresentations and deceptive acts, fueled by his animus toward the Plaintiff and his family members. His actions constitute grave violations of his contractual and fiduciary duties to the Plaintiff, and Defendant must be held accountable."
A Trump spokesperson said in a statement, "The lawsuit and the many wrongdoings by Michael Cohen -- a convicted felon -- stand for themselves, and have been admitted to by Cohen himself through his falsehood-filled books, podcasts, and constant media appearances.”
Fox News was first to report the lawsuit.
In filing his lawsuit now, Trump could be seen as trying to dismantle Cohen's credibility at a time when Cohen has become an important witness in the criminal case against Trump in New York.
The former president pleaded not guilty last week to 34 felony counts of falsifying business records, which involve reimbursements to Cohen for hush money paid to porn actress Stormy Daniels in the closing weeks of the 2016 campaign.
Cohen told ABC's "Good Morning America" last month -- prior to the indictment -- that he was willing to testify in any possible case.
"The facts are the facts. The truth is the truth and the truth will always rise so I'm not worried about anything that they want to come at me with," he said.
Cohen served prison time after he pleaded guilty to federal charges that included campaign finance violations related to the hush payment. He was sentenced to three years but received early release in November 2021.
ABC News' Libby Cathey contributed to this report.