Ronda Rousey Become First Female UFC Fighter

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.