Ronda Rousey just made MMA history ... becoming the very first female fighter to join the UFC . Rousey -- a former U.S. Olympian -- has officially joined Dana White's MMA organization ... and automatically becomes the 135 lb women's champion. We're told 25-year-old Ronda will defend her title in 2013 -- possible opponents include Sara McMann, Cris Cyborg and rival Miesha Tate. Rousey has been fighting in Strikeforce since last year and emerged as a superstar ... beating the living crap out of just about everybody she's faced. In fact, she's undefeated. Just check out this highlight reel. (WARNING: SHE SNAPS SOME CHICK'S ELBOW IN THE VIDEO) Read more: http://www.tmz.com#ixzz2BjpIXKpe