The feud between two of America’s best-known attorneys has escalated, with Alan Dershowitz filing a defamation suit against David Boies.

In a countersuit in New York State Supreme Court in Manhattan, Mr. Dershowitz alleged defamation and intentional infliction of emotional distress.

Filed Monday, the suit says Mr. Boies, his firm Boies Schiller Flexner LLP and Jeffrey Epstein accuser Virginia Giuffre tried to extort money from wealthy and powerful men, including billionaire Leslie Wexner, for private settlements.

The countersuit comes in response to Mr. Boies’s defamation suit against Mr. Dershowitz, filed in November in New York. Mr. Boies cited news articles he said amounted to a campaign of disparagement, including a Washington Post interview where Mr. Dershowitz referred to Boies Schiller as “the law firm of extortion, subornation of perjury and other crimes.”

Boies Schiller Flexner isn’t a defendant in the suit. Neither Mr. Boies nor the firm responded to a request to comment.