The justices unanimously 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 US constitution's First Amendment protection against government abridgement 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 January 10, just nine days before the deadline set under the law.
TikTok, ByteDance and some of the app's users have challenged a move to ban the platform. (AP PHOTO)
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 people, 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 US 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 US national security, with China seeking to amass vast quantities of sensitive data on US citizens 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 weaponise 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 December 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.
