Abstract: The waterbed sheet assembly 25 includes a top sheet 28, and a bottom sheet 26, with the sheets sewn together along their bottom edges. A corner pocket structure 30 is formed on the corners of the bottom sheet, and each corner pocket structure includes a corner pocket panel 31 sewn along its right angle edges 32 and 33to the adjacent side and end edges of the sheet, with a sleeve 40 attached longitudinally along the diagonal free edge 34 of the corner pocket structure. A bar 42 is inserted into the sleeve 40, with the bar extending diagonally across and beneath the corner of the waterbed mattress. The weight of the water in the mattress holds the bar and pocket structure in place beneath each corner of the mattress.