Tumblr will ban all adult content from its platform beginning Dec. 17.

The new policy was announced just weeks after its iOS app was temporarily pulled from the App Store for child pornography. It marks a dramatic shift for the platform, which in the past has embraced all varieties of NSFW content including porn.

That will no longer be the case after Dec. 17. The many Tumblr sites that already have explicit content will be sent notices that their posts have been flagged and the posts will be switched to a private mode that makes them only viewable to the account holder following the deadline.

"Adult content primarily includes photos, videos, or GIFs that show real-life human genitals or female-presenting nipples, and any content—including photos, videos, GIFs and illustrations—that depicts sex acts," Tumblr writes of its new policy.

"Written content such as erotica, nudity related to political or newsworthy speech, and nudity found in art, such as sculptures and illustrations, are also stuff that can be freely posted on Tumblr."

Other exceptions include images of breastfeeding and health-related photos, like those showing mastectomies.

But the change is still a dramatic shift for the company, which has long been known as one of the few mainstream social media outlets where just about anything goes. Though Tumblr has had some content rules in the past, like those prohibiting harassment and spam, it's never had a wholesale ban on explicit content.

Tumblr's openness to porn has not only given rise to a number of communities who eagerly share and celebrate adult content, it has given the site a reputation as the social media site most known for porn and NSFW content. In fact, according to data from analytics site SimilarWeb, adult content is among the top interests for Tumblr users.

The policy change is the latest sign that Tumblr has been trying to adapt its policies following Yahoo's acquisition by Verizon. Last year, there was an outcry among young Tumblr users when the site introduced a new mandatory "safe mode" that blocked all NSFW content to minors with no option to opt out. And Tumblr's policies were again in the spotlight last month when its app was temporarily pulled from Apple's App Store for child pornography.

"We spent considerable time weighing the pros and cons of expression in the community that includes adult content. In doing so, it became clear that without this content we have the opportunity to create a place where more people feel comfortable expressing themselves," Tumblr CEO Jeff D’Onofrio wrote in a post explaining the change.

"Bottom line: There are no shortage of sites on the internet that feature adult content. We will leave it to them and focus our efforts on creating the most welcoming environment possible for our community."