Elon Musk’s X sues advertising group over boycott that allegedly cost the platform billions

X alleged companies including CVS Health, Mars, and Unilever conspired to withhold billions in advertising dollars to force the platform formerly known as Twitter to maintain certain safety standards, according to the complaint filed Tuesday in Texas’ northern district. The boycott began in November 2022 after Musk acquired Twitter and continues today, the lawsuit said.

X said the coalition violated antitrust laws by having its members agree to a boycott, which X called a “coercive exercise of market power."

The companies cited in the X lawsuit were part of the World Federation of Advertisers group, which has dozens of members. The group has an initiative called the Global Alliance for Responsible Media pushing for harmful content to be removed from social media so they aren’t placed next to ads from their members.

The Global Alliance for Responsible Media noted “the massive economic harm imposed on Twitter by the boycott," the lawsuit said. “The boycott and its effects continue to this day, despite X applying brand safety standards comparable to those of its competitors."

The World Federation of Advertisers and the other defendants didn’t respond to requests for comment.

Advertisers have fled X since Musk bought the platform in 2022, partially in response to Musk’s inflammatory posts. Last year, several large advertisers ditched the platform after the billionaire described an antisemitic post as “the actual truth." At a conference, he said advertisers pulling their ads from X can “go f— yourself."

In multiple X posts on Tuesday, Musk urged companies who have faced advertising boycotts to also file lawsuits.

“We tried peace for 2 years, now it is war," Musk said on X.

Musk has gotten more litigious in recent months. On Monday, he revived his lawsuit against Sam Altman and OpenAI. X has sued watchdog group Media Matters for America, and is bankrolling a lawsuit against Disney. Musk is also facing lawsuits from Twitter employees he laid off and from Don Lemon, who briefly had a partnership deal with X.

X’s chief executive, Linda Yaccarino, has been trying to persuade advertisers to return. With X’s lawsuit, the company is now trying to bring them back by force.

Yaccarino, in a video post addressed to X users, said “no small group should be able to monopolize what gets monetized."

“They conspired to boycott X, which threatens our ability to thrive in the future," said Yaccarino, who was wearing a necklace that said “free speech." “That puts your global town square, the one place you can express yourself freely and openly, at long term risk."

Rumble, a video platform popular with conservatives, said Tuesday it was also filing antitrust lawsuits against companies that withheld advertising money.

X’s ad revenue fell sharply in the year after Musk bought the platform. Recently, though, Musk has touted his own internal metric which he claims shows record usage on X.

Write to Alyssa Lukpat at alyssa.lukpat@wsj.com and Joseph De Avila at joseph.deavila@wsj.com