Montana has become the first US state to ban TikTok, with the law set to take effect next year.
The move comes as debate escalates over the impact and security of the popular video app.
The prohibition signed into law by Governor Greg Gianforte will serve as a legal test for a national ban of the Chinese-owned platform, something that politicians in Washington are increasingly calling for.
"TikTok may not operate within the territorial jurisdiction of Montana," said a copy of the law on the state website.
The ban makes it a violation each time "a user accesses TikTok, is offered the ability to access TikTok, or is offered the ability to download TikTok."
Each violation is punishable by a $10,000 (€9,200) fine every day it takes place.
Under the law, Apple and Google will have to remove TikTok from their app stores and companies will face possible daily fines.
State political leaders have "trampled on the free speech of hundreds of thousands of Montanans who use the app to express themselves, gather information, and run their small business in the name of anti-Chinese sentiment," said ACLU Montana policy director Keegan Medrano.
The ban will take effect in 2024 but be voided if TikTok is acquired by a company incorporated in a country not designated by the United States as a foreign adversary, the law reads.
"Governor Gianforte has signed a bill that infringes on the First Amendment rights of the people of Montana by unlawfully banning TikTok," a spokeswoman for the company said.
"We want to reassure Montanans that they can continue using TikTok to express themselves, earn a living, and find community as we continue working to defend the rights of our users inside and outside of Montana."
The company has gone on record saying that the ban's constitutionality will ultimately be decided by the courts.
The law is the latest skirmish in duels between TikTok and many western governments, with the app already banned on government devices in the US, Canada and several countries in Europe.
The app is owned by Chinese firm ByteDance and is accused by a wide swathe of US politicians of being under the tutelage of the Chinese government and a tool of espionage by Beijing, something the company denies.
Mr Gianforte himself said on Twitter that he signed the ban in order to "protect Montanans' personal and private data from the Chinese Communist Party."
Despite its immense popularity, TikTok has been given an ultimatum by the White House that it split from its Chinese owners or stop operating in America.
Montana's clampdown on TikTok comes as the app faces legislative changes at a national level, including one bill that could give the White House significant new powers to oversee Chinese tech companies.
