Ben Stiller & Owen Wilson -- It's a Walk-Off! Hansel and Derek Crash Paris Fashion Week

Derek Zoolander and Hansel McDonald un-retired for a surprise appearance in Paris today ... treating the crowd at the Valentino show to an epic "walk-off."

Ben Stiller and Owen Wilson stunned the crowd by bringing back the characters for Fashion Week ... as sort of a dry-run for the upcoming "Zoolander" sequel.

Neither said a word -- in true male model fashion -- but worked their magic on the runway.

Finally ... more funding for "The Derek Zoolander Center for Kids Who Can't Read Good and Wanna Learn to Do Other Stuff Good Too."

They're so hot right now.