YouTube said on Wednesday it would start removing content that falsely alleges that widespread voter fraud changed the result of the US election.
The update applies to all new content, including videos from President Donald Trump.
The company had previously labelled potentially misleading election videos, adding links to accurate information.
YouTube said “enough states have certified their election results to determine a President-elect.”
Democrats have criticised YouTube for not doing enough to take down fake news and conspiracy theories on the platform.
Mr Trump and senior Republicans have repeatedly made unsubstantiated claims that the election was “rigged”.
Trump’s lawyers have failed to provide evidence of this.
The announcement comes after a “safe harbor” deadline – which sets a date by which states need to certify the results of the presidential election.
“Yesterday was the safe harbor deadline for the US Presidential election and enough states have certified their election results to determine a President-elect,” said YouTube.
It also said that the move was in line with its historical approach to US presidential elections.
Examples it cited of videos it would now remove were uploads claiming that a presidential candidate won the election due to widespread software glitches or counting errors.
Reuters identified several YouTube channels making money from ads and memberships that were amplifying debunked accusations about voting fraud.
YouTube said that it has taken down 8,000 channels since September, for uploading “harmful and misleading elections-related videos for violating our existing policies”.
The latest move will anger President Trump and many Republicans, many of whom already believe Big Tech is biased against conservatives.
The focus now moves to Twitter and Facebook as to whether they will follow YouTube’s lead.