A rotatable tailgate assembly is provided for installation on a dump bed. The tailgate assembly has support members attachable to the side of the dump bed. Each support member has at least one bearing adjacent at a free end extending rearwardly of the dump bed's rear opening. A tailgate has a cylindrical tailgate shaft with a first end rotatably received within the bearing of a first support member and a second end rotatably received within a bearing of a second support member. The tailgate has a closure portion extending radially away from the tailgate shaft. The closure portion engages the rear edges of the dump bed floor and sides to close off the dump bed opening when the tailgate assembly is in a closed rotational position. The tailgate assembly also comprises a rotation actuator operatively connected to the tailgate shaft for selective rotation thereof.