Abstract: An adjustable, portable utility shelf includes a main tray having a flat work surface and raised sides. Slide guides are disposed along the sides of the main tray, and comprise channels for receiving correlatively shaped and inter-engageable channels on removable auxiliary trays. A clamp assembly includes a base plate rotatably attached to the rear wall of the main tray at its center, and is releasably attachable to a side rail of a ladder. The base plate has a plurality of locating holes in it forming an arc with respect to the rotatable connection between the base plate and the rear wall of the main tray. A tray support bracket attached to the center of the underside of the main tray includes a strut having a locating pin protruding therefrom. The locating pin can be placed in any one of the locating holes, fixing the tray at a selected angular orientation with respect to the base plate, and is positively retained in the selected locating hole by the weight of the tray and its contents.