Abstract: A shelf apparatus configured for temporary attachment to a ladder deployed in a working position includes a planar tray supported at its inboard end by a rung or top plate of the ladder, left and right support arms pivotably attached to opposite sides of the tray near its inboard end, a rigid linkage coupling the tray to the left and right support arms, and an adjustable clamping mechanism for clamping the tray to the rung or top plate. In use with a straight ladder, the tray is horizontally suspended under the inclined frame of the deployed ladder, the support arms are raised to rest against the inclined rails of the ladder above the tray, the rigid linkage vertically supports the tray from the support arms, and the clamping mechanism secures the tray to the rung.