Reddit has banned "fake porn" - imagery and videos that superimpose a subject's face over an explicit photo or video without the person's permission.
The move follows the development of artificial intelligence software that made it relatively easy to create clips featuring computer-generated versions of celebrities' faces.
Reddit had become one of the most popular places to share and discuss so-called deepfake videos.
It also reworded its policy for minors.
The discussion site had been under growing pressure to act after other platforms - including Twitter, Gfycat and Pornhub - introduced their own deepfake bans.
However, it may cause unease among some Reddit users who already feared the platform was becoming less "open" after its closure of two alt-right forums in 2017.
The move coincided with an announcement that Reddit's co-founder Alexis Ohanian was stepping aside from a hands-on role and would instead dedicate most of his time to a venture capital firm.
He did not comment on the new policies but told the Wall Street Journal that he intended to focus on opportunities related to emerging technologies.
"There are clearly things around security and privacy and identity that are going to be dramatically changed through technology, just because it's now possible," he said.
Mr Ohanian will remain a Reddit board member.