The actor, who plays the titular role in the BBC drama, revealed that he was alerted to the "racy" stories by his co-star Martin Freeman, who discovered them while in New Zealand shooting The Hobbit.
"I am startlingly aware of it. I don't go seeking it, but Martin Freeman, who was an absolute luddite on the first series, suddenly came back from New Zealand... and he had this beautiful Mac with him and said, 'Hey, look at this Tumblr'. And I said, 'What? Tumblr? What?'" Cumberbatch recalled in an interview with MTV News.
"He knows more about it than I do and he was showing me some of them. Some of it is really racy, un-viewable even on MTV.
"It's cool. I suppose my bodily proportions are quite flattering. I'm ripped, doing something I wouldn't normally do with my body, or having done to it, involving Watson. So that's as far as I'll hit about that one, but it's all there on the web if you want to find it."
Cumberbatch went on to say that he was surprised to see such a high quality of writing amongst the fan-penned pieces, adding: "I was amazed at the level of artistry; people have spent hours doing it. And there's some really weird cross breeding stuff that goes on.
"The news got out that I was playing Smaug in Hobbit and suddenly there were lots of dragons with purple scarves flying around so it's crazy, it's crazy."