Ronda Rousey BOOKS 1ST UFC FIGHT Main Event at UFC 157

Ronda Rousey has signed a contract to fight in the MAIN EVENT at UFC 157 in February ... the first ever female fight in UFC history ... TMZ has learned.

Sources tell us the 25-year-old Olympic medalist will step in the ring with 28-year-old Liz Carmouche ... a highly-ranked MMA badass who served in the U.S. Marine Corps.

Rousey was named UFC champion (female 135 lbs division) when she signed with the organization -- and her belt will be up for grabs.

The fight is set to go down on Feb. 23 in Anaheim, CA.

FYI -- Carmouche has a 7-2 record and an awesome nickname -- The Girl-rilla.