Ryan Seacrest -- Single Again and Mingling ... With Hot New Chick

Ryan Seacrest has broken up with his girlfriend -- so we've learned -- and is out on the town with a new, attractive blonde chick.

Ryan was leaving the GQ Men of the Year party Thursday night at the Chateau Marmont in West Hollywood and his GF of more than a year-and-a-half, Shayna Terese Taylor, was nowhere in sight. That's because we're told Seacrest said bye bye.