President Trump signed an executive order on Monday to delay enforcing a federal ban of TikTok for 75 days, even though the law took effect on Sunday and it is unclear that such a move could override it.

The order, one of Mr. Trump’s first acts after taking office, instructs the attorney general not to take any action to enforce the law so that his administration has “an opportunity to determine the appropriate course forward.” The order is retroactive to Sunday.

As he signed the order, Mr. Trump told reporters that “the U.S. should be entitled to get half of TikTok” if a deal for the app is reached, without going into detail. He said he thought TikTok could be worth a trillion dollars.

The order could immediately face legal challenges, including over whether a president has the power to halt enforcement of a federal law. Companies subject to the law, which forbids providing services to Chinese-owned TikTok, may determine that the order does not provide a shield from legal liability.

The federal law banning TikTok, which is owned by ByteDance, mandated that the app needed to be sold to a non-Chinese owner or it would be blocked. The only workaround provided by the law is a 90-day extension if a likely buyer is found. Even then, it is unclear if that option is viable, given that the law is already in effect. The law also restricts how much of a TikTok stake can remain under foreign ownership.

By seeking to override the federal law, Mr. Trump raised serious questions about the limits of presidential power and the rule of law in the United States. Some lawmakers and legal experts have expressed concerns about the legality of an executive order, particularly in the wake of a Supreme Court ruling that upheld the law on Friday and the national security concerns that prompted legislators to draft it in the first place.

Former President Joseph R. Biden Jr. had signed the law, which passed overwhelmingly in Congress last year, forcing ByteDance to sell TikTok or face a ban. TikTok had faced security concerns that the Chinese government could use it to spread propaganda or collect U.S. user data. The law levies financial penalties on app stores and cloud computing providers unless they stop working with the app.

TikTok briefly went dark for U.S. users over the weekend, but returned Sunday following Mr. Trump’s social media announcement that he was planning an executive order. While the app was working again for people who have already downloaded it, it vanished from Google’s and Apple’s app stores on Saturday and remained unavailable on Monday.

Mr. Trump’s efforts to keep TikTok online have major implications for its users. The app has reshaped the social media landscape, defined popular culture and created a living for millions of influencers and small businesses that rely on the platform.

In the executive order, Mr. Trump said that his constitutional responsibilities include national security. It says he wants to consult with advisers to review the concerns posed by TikTok and the mitigation measures the company has taken already.

The administration will “pursue a resolution that protects national security while saving a platform used by 170 million Americans,” according to the order, which called the law’s timing “unfortunate.”

The attorney general will send letters to companies covered by the law to tell them “that there has been no violation of the statute” and they won’t be held liable for providing services to TikTok during the 75 days, the order said.

That might not be enough reassurance, some legal experts said.

“I don’t think it’s consistent with faithful execution of the law to direct the attorney general not to enforce it for a determinate period,” said Zachary Price, a professor at the University of California College of the Law, San Francisco. “And even if that’s OK, the president doesn’t have the authority to eliminate the law itself and remove liability for the people who violate it while it’s not being enforced.”

TikTok and Apple did not immediately respond to requests for comment. Google declined to comment.

TikTok’s ties to China have long raised national security concerns, including with Mr. Trump. Near the end of his first term in 2020, Mr. Trump issued an executive order that would bar app stores from making TikTok available for download. He then pushed for an American company to buy the app, but those efforts fizzled when he lost re-election.

Last year, the effort was revived by Congress and Mr. Biden signed it into law in April. The law targeted app stores, like those run by Apple and Google, and cloud computing companies. It said those companies could not distribute or host TikTok unless the app was sold to a non-Chinese owner by Jan. 19.

Mr. Trump then reversed positions. He joined the app in June and said on television in March that there are young people who would go “crazy” without TikTok.

“I guess I have a warm spot for TikTok that I didn’t have originally,” Mr. Trump said as he signed executive orders Monday evening.

TikTok challenged the law in federal court, saying it impeded its users’ rights to freedom of speech as well as the company’s own First Amendment rights. The Court of Appeals for the D.C. Circuit upheld the law in December. TikTok appealed to the Supreme Court, which on Friday also upheld the law.

TikTok and some Democrats made a last-ditch effort to stop the law from taking effect. But on Saturday, TikTok stopped operating in the United States and disappeared from Apple’s and Google’s app stores a few hours before midnight. Users grieved its disappearance.

On Sunday morning, Mr. Trump announced on Truth Social that he would “issue an executive order on Monday to extend the period of time before the law’s prohibitions take effect, so that we can make a deal to protect our national security.” He said he would not punish companies that had violated the law to keep the app online.

Hours later TikTok restored its service to U.S. users and welcomed them back with a message: “As a result of President Trump’s efforts, TikTok is back in the U.S.!”

As he signed executive orders in the Oval Office, Mr. Trump was asked why he had changed his mind about the app.

“Because I got to use it,” he said.

Tripp Mickle and Nico Grant contributed reporting.

Sapna Maheshwari contributed reporting