Abstract: A waterbed mattress adapted to fit within a rigid frame comprises a water-containing main chamber and at least one supplemental chamber along the perimeter of the main chamber for containing a fluid giving structural rigidity to the perimeter, particularly along the upper boundary abutting the frame. The supplemental chamber is established by an imperforate flexible membrane internally spanning the mattress corner between the top panel and the adjacent side panel. The cross sectional length of the membrane is not greater than the dimension of the adjacent top and side panel segments which constitute the outer walls of the chamber.