Users in the country will be stopped from downloading the video app as well as WeChat.
If a deal is not struck by 12 November, the app will be fully banned, and using as well as downloading the app will be illegal, according to the order from the US Department of Commerce.
Donald Trump and the US government have threatened to ban the app amid accusations that their Chinese owners mean that American data could be put at risk.
People who already have the app installed will still be able to use it as normal after 20 September. But they will not be able to download new updates, which could quickly mean the app’s functionality will break, since developers will not be able to fix bugs or make changes.
Officials will also not look to ban Google and Apple from offering the app through their stores in other countries.
After 12 November, it will be illegal in the US not only to distribute TikTok through app stores but also to provide the underlying internet infrastructure that powers it, or to allow its code to be accessible. That would effectively amount to a complete ban, with users unable to access the app at all.
WeChat will be entirely banned from 20 September, the order said.
The decision comes after threats from the US government that the app would be banned if it could not be sold to a US company.
Silicon Valley tech firm Oracle has been reported to have won that battle, but the deal is yet to go through. The ban could be scrapped if that deal is successfully completed, Reuters reported, citing US commerce officials.
Donald Trump said yesterday that his administration had spoken to Walmart and Oracle about a possible deal, but that there had been no substantial change in the situation.
“We’re making a decision. We spoke today to Walmart, Oracle. I guess Microsoft is still involved,” Trump told reporters at the White House before leaving for a visit to Wisconsin.
“We’ll make a decision, but nothing much has changed. We’ll make a decision soon.”
In early August, Mr Trump signed an executive order that set a deadline of 20 September for the sale of TikTok. In recent remarks, the president had said there would be no extension of that order – though he also falsely suggested that the deadline would come on 15 September.
The executive order said that if the deadline was reached and the app was still under Chinese ownership, the US would ban “any transaction by any person” with Bytedance, TikTok’s owners. Legal experts have speculated on whether the ruling would legitimately allow a ban, but the Trump administration has been clear that was the intention of the document.
The executive order alleged that apps “developed and owned by companies in the People’s Republic of China (China) continues to threaten the national security, foreign policy, and economy of the United States” before going on say specifically that “at this time, action must be taken to address the threat posed by one mobile application in particular, TikTok”.
It accused the app capturing “vast swaths of information from its users, including Internet and other network activity information such as location data and browsing and search histories” as well as censoring content on behalf of the Chinese government and helping with the country’s disinformation campaigns.
TikTok and its parent company Bytedance – which offers a separate version of the app in China – have repeatedly denied those accusations.