Ronda Rousey -- Becomes 1st Female Fighter to Join UFC

Ronda Rousey just made MMA history ... becoming the very first female fighter to join the UFC ... TMZ has learned.

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)

And there's more ... sources say Strikeforce, which airs on Showtime, will be running its last event in January before folding up shop for good. A rep for Strikeforce had no comment.

It's a pretty crazy turn of events -- considering last year, Dana told TMZ women would "NEVER" fight in the UFC.