Abstract: The invention is a utility tray assembly that is capable of being mounted on a support member of a ladder, such as a shelf or a step. The assembly includes a tray having parallel side walls, parallel end walls disposed normal to the side walls, and a bottom defining a plane and disposed normal to the side walls and the end walls. The end walls have slots therein extending in a direction parallel to the plane of the bottom. The assembly further includes a strap having a body portion and opposing end portions. Each of the end portions includes a slot extending in a direction parallel with the longitudinal axis of the strap. A fastener is associated with each end wall and is insertable through the slots in the end wall and the strap for securing the end portions of the strap to the tray end walls in such a manner that the support member of the ladder is captured between the body portion of the strap and the undersurface of the tray bottom.