Reports are flooding in from across the web that affected accounts are producing a series of offensive posts and spreading a worm virus to other users.
The organisation responsible - known as the "Gay N***er Association of America" - has previously targeted the campaign website of president Barack Obama and trolled various media outlets.
Technology site The Verge was among those whose Tumblr was affected by the breach."Yes, our Tumblr appears to have been hacked," it wrote on Twitter. "We're taking care of this - and we recommend staying away from those nasty links."
Users whose blogs are affected are advised to use the Mega-Editor feature to mass delete the posts and change their passwords.
The hack has not affected all Tumblr accounts and is currently being dealt with.