The move comes in response to so-called "challenges" that have sometimes resulted in death or injury.
The Google-owned video sharing site said such material had “no place on YouTube”.
However, the firm appears to be failing to enforce its existing rules on harmful content.
A Buzzfeed report detailed how images depicting, or alluding to, bestiality were still appearing on the site - despite a pledge last April to remove the material.
Some of the videos had attracted many millions of views. YouTube said it “worked to aggressively enforce our monetisation policies to eliminate the incentive for this abuse”.
But enforcing its new rules on pranks may prove even more difficult, given ambiguity over what may or may not be considered harmful.
“YouTube is home to many beloved viral challenges and pranks,” a message added to the site’s FAQ section read.
“That said, we’ve always had policies to make sure what’s funny doesn’t cross the line into also being harmful or dangerous.
“Our Community Guidelines prohibit content that encourages dangerous activities that are likely to result in serious harm, and today clarifying what this means for dangerous challenges and pranks.”
From now on, the site said it would not allow videos that featured “pranks with a perceived danger of serious physical injury".
This includes pranks where someone is tricked into thinking they are in severe danger, even if no real threat existed.