Abstract: A bedding sheet assembly, particularly adapted for waterbeds but also usable with other types of mattresses, having a novel mattress belt (1), a first or bottom sheet (2), and a second or top sheet (3). The mattress belt (1) has sides (10) that fit around the perimeter of a mattress (4), a small lip on the bottom (8) to fit under the mattress and a top lip (9) to fit on top of the mattress along its sides. The top lip (9) of the mattress belt contains fastening material (7) for securing the bottom sheet thereto by matching fastening material (17) on the underside of the bottom sheet. A flap (12) attached to the foot end of the mattress belt has fastening material (11) for securing the top sheet (3) thereto. The bottom and top sheets may have ruffled flaps (25 and 18) to fit over the sides of the mattress belt to keep the mattress belt clean and attractive.