A retractable U-shaped ladder assembly preferably for a craft such as a watercraft has a rotatably retractable ladder which is attached to the craft by brackets. A torsion spring is used to bias the ladder into its retracted position. While the ladder is in an in-use position, the ladder is locked into place against rotation by a locking mechanism. The locking mechanism includes a bent plate which presses against four rings that are fixed to a rod, which rod extends from each bracket. Each ring has a notch that is located in the same position. In the use position, the bent plate will fit into the notches of the rings and lock the step. When the user is finished using the ladder, the user can manually unlock the step by pressing the bent plate. The bent plate moves out of engagement with the notches in the rings, thereby allowing the torsion springs in the brackets to rotate the step to the retracted position. In another embodiment, the bent plate is formed as a U-shaped cover plate, and springs for retracting the ladder are positioned to the inside of the brackets. In a further embodiment, the cover plate is made of plastic and molded with two sleeves for pivotably mounting on pins. In addition, the rod is made of plastic and unitarily molded with the rings.