Elon Musk claimed late Monday that Apple gave preferential treatment to OpenAI, a prominent competitor of his artificial intelligence company, and he threatened to sue the consumer tech giant for “an unequivocal antitrust violation.”

Mr. Musk’s A.I. company, xAI, released a new version of its chatbot, Grok, last month. Mr. Musk has recently posted on his social media platform, X, about how well his chatbot was doing with rankings in Apple’s App Store.

But on Monday, Mr. Musk started posting that Apple was intentionally favoring OpenAI instead. Apple has a partnership with OpenAI to integrate its chatbot, ChatGPT, into Apple products.

“App Store, why do you refuse to put either X or Grok in your ‘Must Have’ section when X is the #1 news app in the world and Grok is #5 among all apps?” Mr. Musk wrote. “Are you playing politics?”

Hours later, Mr. Musk posted that “Apple is behaving in a manner that makes it impossible for any AI company besides OpenAI to reach #1 in the App Store.”

He added that “xAI will take immediate legal action.”

Mr. Musk frequently turns to legal threats against competitors and critics over perceived injustice, sometimes resulting in lawsuits and other times fizzling out after a few social media posts. As of Tuesday afternoon, xAI did not appear to have filed a lawsuit in federal court.

Representatives for xAI and Apple did not respond to requests for comment.

A fact-checking feature run by X users, called Community Notes, added a note to Mr. Musk’s post that said chatbot apps made by companies other than OpenAI had reached the No. 1 spot on the App Store.

Mr. Musk has feuded with OpenAI and Apple before. Mr. Musk previously criticized Apple in 2022, when he said the company had threatened to remove his social media app from its App Store altogether after he relaxed content moderation rules. Apple’s policies prohibit apps that include hateful speech or content.

Mr. Musk later met with Tim Cook, Apple’s chief executive, and clarified it was a “misunderstanding.”

Mr. Musk co-founded OpenAI, but left the company in 2018, citing disagreements with the other co-founders over the company’s direction. He sued OpenAI last year, claiming that the company and two of its founders, Sam Altman and Greg Brockman, had breached its founding contract by putting commercial interests ahead of the public good.

(The New York Times has also sued OpenAI for copyright infringement. The company has denied wrongdoing.)

Mr. Altman, who remains the chief executive of OpenAI, has denied the allegations.