The Upright Citizens Brigade Theatre or UCB Theatre is an improvisational theatre and associated UCB Training Center with locations in Chelsea, New York, the East Village, New York and Hollywood, California..

The theater was founded by Matt Besser, Amy Poehler, Ian Roberts and Matt Walsh, a group of comedians who perform as the Upright Citizens Brigade. The troupe had a show on Comedy Central from 1998 to 2000 and were originally trained by Del Close at Chicago's ImprovOlympic.

The Upright Citizens Brigade Theatre Training Center teaches longform improv, sketch writing, and various other comedy skills. The training center's philosophy of improv is based largely on the teachings of Del Close, with a strong emphasis on the "game" of the scene. The primary improvisational form is "The Harold", and the theater in all its incarnations has had a group of "Harold Teams", house teams that perform regularly. Teams are composed of people who have completed training at the Upright Citizens Brigade Theatre Training Center, and either auditioned or were asked to be performers.