Vatican City -- Does Someone There Watch Porn? ... Is the Pope Catholic?

Vatican City has at least one porn lover on the loose -- someone who's frequently logging on to a popular porn website ... and TMZ has learned there's a good chance it's a woman.

The site is called sssh.com ... which bills itself as porn for "open-minded women who enjoy exploring their sexuality" -- and a rep for the site tells us a routine check of data revealed the IP address of one their members leads back to the Vatican.

We're told the user (or users) visited the site at least once a month for the past year, with November 2013 being an especially busy month ... with 19 visits!

The rep for sssh.com tells TMZ, "It just shows that a healthy interest in erotica is something that can be found anywhere -- even within the Vatican."