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.
"We will continue to challenge the unjust executive order, which was enacted without due process and threatens to deprive the American people and small businesses across the US of a significant platform for both a voice and livelihoods."
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".