The legislation is the most extreme prohibition of the app in the nation and will almost certainly face legal challenges.

The governor of Montana, Greg Gianforte, signed a bill on Wednesday to ban TikTok from operating inside the state, the most extreme prohibition of the app in the nation and one that will almost certainly be challenged in court. The ban will take effect on Jan. 1.

“Today, Montana takes the most decisive action of any state to protect Montanans’ private data and sensitive personal information from being harvested by the Chinese Communist Party,” Mr. Gianforte, a Republican, said in a news release.

To protect Montanans’ personal and private data from the Chinese Communist Party, I have banned TikTok in Montana.— Governor Greg Gianforte (@GovGianforte) May 17, 2023

The Montana Legislature introduced the bill in February, leading to months of debate. The proposal, which would affect everyday users of the popular short-form video app, significantly escalated a national rush to ban TikTok on government devices based on concerns about the company’s ownership by the Chinese company ByteDance. The battle over the bill offered a glimpse of what the United States might encounter nationally if lawmakers or the White House attempt a nationwide ban of TikTok, which has been floated in recent months.

TikTok, which says it has 7,000 employees in the United States, has been fighting back in the state for months. It has run ads featuring Montana small businesses that use TikTok and given prewritten emails to users so they could contact Mr. Gianforte about opposing the bill.

The legislation prohibits mobile app stores, like those run by Apple and Google, from offering TikTok within the state. A trade group funded by Apple and Google has said in recent months that it is impossible for the companies to prevent access to TikTok in a single state.

“Governor Gianforte has signed a bill that infringes on the First Amendment rights of the people of Montana by unlawfully banning TikTok, a platform that empowers hundreds of thousands of people across the state,” Brooke Oberwetter, a spokeswoman for TikTok, said in a statement on Wednesday. Montanans, she added, can keep using the app “as we continue working to defend the rights of our users inside and outside of Montana.”

Under the legislation, TikTok could face fines if it continues operating in the state, as could Apple and Google if they allow people to download the app.

Apple and Google didn’t immediately return requests for comment.

The battle in Montana erupted during a period of intense national scrutiny on TikTok, which boasts more than 150 million U.S. users. Lawmakers and intelligence officials have said TikTok, because of its ownership, could put sensitive user data into the hands of the Chinese government, pointing to laws that allow Beijing to secretly demand data from Chinese companies and citizens for intelligence gathering.

They have also expressed concern that the app, which is especially popular with teenagers and people in their 20s, could be used to spread propaganda. Congress grilled Shou Chew, TikTok’s chief executive, for roughly five hours at a March hearing that focused largely on the app’s Chinese ownership.

TikTok says it has never been asked to provide, nor has it provided, any U.S. user data to the Chinese government. The company has proposed a detailed plan for operating in the United States that it says should allay national security concerns and fears of misinformation, but the plan has not yet been approved by the Biden administration, leaving TikTok and its future in limbo.

Free speech groups were quick to respond to the Montana ban. The American Civil Liberties Union said on Wednesday that the legislation “flouts the First Amendment.”

“The government cannot impose a total ban on a communications platform like TikTok unless it is necessary to prevent extremely serious, immediate harm to national security,” the group said in a statement. “But there’s no public evidence of harm that would meet the high bar set by the U.S. and Montana Constitutions, and a total ban would not be the only option for addressing such harm if it did exist.”

The Montana bill says the ban will be void if TikTok is acquired by, or sold to, a company that is not incorporated in a country “designated as a foreign adversary.”