Abstract: A convertible water bed comprising a bed frame, a water-filled mattress supported on the bed frame, a movable flexible mattress support member slidably connected at two opposite edges to side boards of the bed frame for movement between a use position above the water-filled mattress and a storage position below the water-filled mattress, and a flexible conventional mattress attached to the movable mattress support member for movement therewith between the use and storage positions, so that either the water mattress or the conventional mattress can be used by a simple manipulation of the bed assembly. The flexible support member in the use portion may lie on the upper surface of the water mattress so that the latter can be used as a lower support, in the nature of a box spring, for the conventional mattress.