Donald J. Trump posted on X Monday morning, handing a victory to Elon Musk, the social media platform’s owner, who for months has urged the former president to use the site again.

“They want to silence me because I will never let them silence you,” Mr. Trump said in a campaign video posted to his account. “They’re not coming after me. They’re coming after you.”

Mr. Trump shared the video ahead of a scheduled interview with Mr. Musk, which was set to take place on Spaces, a livestreaming feature on X, at 8 p.m. Eastern on Monday. He followed it up with several posts, including one that said, “Are you better off now than you were when I was president? Our economy is shattered. Our border has been erased. We’re a nation in decline.”

Mr. Trump’s return to X after nearly a year of silence could provide a needed boost to the platform, which has faced increased competition from a rash of new social media platforms, including the former president’s Truth Social. X has also struggled to overcome skepticism from advertisers, especially after Mr. Musk sued an influential advertising coalition last week.

But if Mr. Trump returns as a power user of the site, which helped fuel his political rise, it could attract new interest from political advertisers. During his time in the Oval Office, Mr. Trump used the site as a megaphone to issue policy directives and pummel his rivals. At the beginning of his presidency, he posted about nine times a day — but that pace accelerated throughout his time in office.

Mr. Trump was banned from X, then known as Twitter, in 2021 after his supporters stormed the U.S. Capitol in an attempt to halt the certification of the election. Twitter determined that Mr. Trump’s posts to his 88 million followers spurred violence and his account was subsequently shut down.

Mr. Musk, who acquired the platform in 2022, reinstated the former president’s account shortly after buying the company. Mr. Musk urged Mr. Trump to come back to X, believing his presence would attract fresh engagement on the struggling site.

But Mr. Trump had since formed Truth Social, where he has 7 million followers. He’s contractually obligated to share social media posts there before turning to other platforms, except ones about his campaign and politics.

Shares in Trump Media & Technology Group, the parent company of Truth Social, opened lower on Monday and fell further after Mr. Trump began posting on X, closing down 5 percent. Mr. Trump owns a 60 percent stake in the company, but he can begin selling his shares on Sept. 19, when a company lawyer has said a six-month lockup period will expire for all corporate insiders.