WASHINGTON, Jan 17 (Reuters) - The U.S. Supreme Court

ruled against TikTok on Friday in its challenge to a federal law

that would have required the popular short-video app to be sold

by its Chinese parent company ByteDance or banned in the United

States on Jan. 19.

The justices ruled that the law, passed by an overwhelming

bipartisan majority in Congress last year and signed by

Democratic President Joe Biden, did not violate the U.S.

Constitution's First Amendment protection against government

abridgment of free speech. The justices overturned a lower

court's decision that had upheld the measure after it was

challenged by TikTok, ByteDance and some of the app's users.

The Supreme Court acted speedily in the case, having held

arguments on Jan. 10, just nine days before the deadline set

under the law. The case pitted free speech rights against

national security concerns in the age of social media.

TikTok is one of the most prominent social media platforms

in the United States, used by about 270 million Americans -

roughly half the country's population, including many young

people. TikTok's powerful algorithm, its main asset, feeds

individual users short videos tailored to their liking. The

platform presents a vast collection of user-submitted videos,

often under a minute in duration, that can be viewed with a

smart phone app or on the internet.

China and the United States are economic and geopolitical

rivals, and TikTok's Chinese ownership for years has raised

concerns among American leaders. The TikTok fight has unfolded

during the waning days of Biden's presidency - Republican Donald

Trump succeeds him on Monday - and at a time of rising trade

tensions between the world's two biggest economies.

The Biden administration has said the law targets control of

the app by a foreign adversary, not protected speech, and that

TikTok could continue operating as-is if it is freed from

China's control.

During arguments in the case, Justice Department lawyer

Elizabeth Prelogar said Chinese government control of TikTok

poses a "grave threat" to U.S. national security, with China

seeking to amass vast quantities of sensitive data on Americans

and to engage in covert influence operations. Prelogar said

China compels companies like ByteDance to secretly turn over

data on social media users and carry out Chinese government

directives.

TikTok's immense data set, Prelogar added, represents a

powerful tool that could be used by the Chinese government for

harassment, recruitment and espionage, and that China "could

weaponize TikTok at any time to harm the United States."

The law was passed last April. Biden's administration

defended it in court. TikTok and ByteDance, as well as some

users who post content on the app, challenged the measure and

appealed to the Supreme Court after losing on Dec. 6 at the U.S.

Court of Appeals for the District of Columbia Circuit.

Trump's opposition to the ban represents a reversal in

stance from his first term in office when he aimed to prohibit

TikTok. Trump has said he has "a warm spot in my heart for

TikTok," opining that the app helped him with young voters in

the 2024 election.

In December, Trump asked the Supreme Court to put the law on

hold to give his incoming administration "the opportunity to

pursue a political resolution of the questions at issue in the

case." But while Trump has vowed to "save" TikTok, many of his

Republican allies supported the ban.

Mike Waltz, Trump's incoming national security adviser, said

on Thursday the new administration will keep TikTok alive in the

United States if there is a viable deal. Waltz said the incoming

administration would "put measures in place to keep TikTok from

going dark," and cited a provision in the law allowing for a

90-day extension if there is "significant progress" toward a

divestiture.

Senate Democratic leader Chuck Schumer said on Thursday that

TikTok should be given more time to find an American buyer and

that he would work with the Trump administration "to keep TikTok

alive while protecting our national security."

TikTok CEO Shou Zi Chew will attend Trump's inauguration on

Monday, seated among other high-profile invitees.

TikTok has said the law endangers the First Amendment rights

not only of it and its users, but also of all Americans. TikTok

has said that the ban would hit its user base, advertisers,

content creators and employee talent. TikTok has 7,000 U.S.

employees.

Noel Francisco, the lawyer for TikTok and ByteDance, told

the Supreme Court that the app is "one of America's most popular

speech platforms," and said that the law would require it to "go

dark" unless ByteDance executes a qualified divestiture.

TikTok plans to shut U.S. operations of the app on Sunday

barring a last-minute reprieve, people familiar with the matter

told Reuters on Wednesday.

Francisco said the U.S. government's real target with this

law is speech - specifically a fear that Americans could be

"persuaded by Chinese misinformation." But the First Amendment

leaves that up to people of the United States, not the

government, Francisco said.

The law bars providing certain services to TikTok and other

foreign adversary-controlled apps including by offering it

through app stores such as Apple ( AAPL ) and Alphabet's

Google, effectively preventing its continued U.S. use

absent divestiture.