The company said it would ban material targeting a person or group with conspiracy theories that have been used to justify violence, such as QAnon.
Twitter and Facebook have already taken similar steps to root out QAnon.
The QAnon movement alleges US President Donald Trump is battling a cabal of elite Satan-worshipping paedophiles.
The FBI last year issued a warning about "conspiracy theory-driven domestic extremists" and designated QAnon a potential domestic extremist threat.
In a blog post on Thursday, YouTube said it had "removed tens of thousands of QAnon videos and terminated hundreds of channels" under its existing content rules.
But citing challenges in managing "shifting and evolving" content, YouTube said it was necessary to take "another step in our efforts to curb hate and harassment".
"Today we're further expanding both our hate and harassment policies to prohibit content that targets an individual or group with conspiracy theories that have been used to justify real-world violence," it said.
One example, YouTube said, "would be content that threatens or harasses someone by suggesting they are complicit in one of these harmful conspiracies".
YouTube's new rules, which represent an expansion of the platform's hate-speech policy, come three years after QAnon first appeared.
The movement has been linked to numerous false claims that have spread online, with the baseless accusations most commonly directed at Democrats.
President Trump has used his Twitter feed to promote unsubstantiated allegations pushed by accounts linked to the QAnon conspiracy theory.
During a meet-the-voters TV event Thursday, the president was asked about the conspiracy theory and replied: "I know nothing about QAnon."
After the moderator pushed him on the answer, he said: "I know nothing about it, I do know they are very much against paedophilia, they fight it very hard."
Supporters of President Trump have been seen holding flags and wearing T-shirts bearing the QAnon symbol at rallies ahead of the 3 November presidential election.
On Wednesday, former US President Barack Obama reprimanded his successor, Mr Trump, and Republicans for giving QAnon a platform.
"Trump is a symptom of [misinformation] and an accelerant to it," Mr Obama told the Pod Save America podcast. "When you look at insane conspiracy theories like QAnon seeping into the mainstream of the Republican Party, what that tells you is that there are no more guardrails within that media ecosystem."
QAnon is a wide-ranging unfounded conspiracy theory that President Trump is battling a clandestine "deep state" network of political, business, media and entertainment elites, often involving Satanic plots and child trafficking.
QAnon began in October 2017 on the anonymous message board 4chan. An anonymous user left a series of posts on 4chan claiming to be involved in a secret investigative team led by President Trump.
They claimed to have top security access within the US government, known as "Q clearance" - hence the name QAnon.
This user's messages became known as "Q drops" or "breadcrumbs", often written in cryptic language peppered with slogans, pledges and pro-Trump themes.
QAnon followed on from the "pizzagate" saga in 2016 - a fake theory about Democratic politicians running a paedophile ring out of a Washington pizza restaurant.
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