The Department of Commerce said it would bar people in the US from downloading the messaging and video-sharing apps through any app store on any platform.
The Trump administration says the companies threaten national security and could pass user data to China. But China and both companies deny this. WeChat will effectively shut down in the US on Sunday, but people will still be able to use TikTok until 12 November, when it could also be fully banned.
TikTok said it was "disappointed" in the order and disagreed with the commerce department, saying it had already committed to "unprecedented levels of additional transparency" in light of the Trump administration's concerns.
Tencent - the owner of WeChat - said the announced restrictions were "unfortunate", but said they would continue talks with the US government "to achieve a long-term solution".
The ban order from the Department of Commerce follows President Trump's executive orders signed in August. It gave US businesses 45 days to stop working with either Chinese company. If a planned partnership between US tech firm Oracle and TikTok owner ByteDance is agreed and approved by President Trump, the app will not be banned.
What does the order say?
"At the president's direction, we have taken significant action to combat China's malicious collection of American citizens' personal data," the US Department of Commerce Secretary Wilbur Ross said in a statement. The department acknowledged that the threats posed by WeChat and TikTok were not identical but said that each collected "vast swathes of data from users, including network activity, location data, and browsing and search histories".
The order means that from Sunday, people will not be able to use WeChat to transfer funds or process payments to or from people in the US. But TikTok users will still be able to use their app virtually as normal, although they will not be able to download new updates.
"The President has provided until November 12 for the national security concerns posed by TikTok to be resolved," the commerce department said. After this point, some technical transactions will be banned on the app and functionality will be affected.
TikTok's owner, Bytedance, now has to try to find a more palatable deal - and this may be tough. It's been reported that China would rather TikTok be closed down than be forced to sell to the US.
In truth, even if a TikTok download ban comes into force, it will still run on peoples' phones. It won't start technically degrading until 12 November. So there's still plenty of life left in this story.
