Abstract: A waterbed construction having an inner rigid frame member and a supported flexible bottom member for holding and laterally supporting a water filled bladder or vinyl mattress. The inner rigid frame member and bladder are covered and interlocked by a cushioned "cap" having rigid side walls and a flexible top member. The cushioned cap fits closely over the inner rigid frame member in much the same manner as a cover fits over a box. This arrangement provides additional strength for containing the lateral forces generated by the water filled bladder and at the same time provides maximum sitting comfort. In addition, the use of a close fitting cap having outer rigid side walls in combination with an inner rigid walled frame member, provides a sealed container for the bladder and virtually eliminates the possibility of water loss in the event the bladder develops a leak.