A water bed mattress made of a water proof, flexible material that has tensile strength, comprising at least three chambers provided with valves, one chamber being divided into an upper and a lower chamber portion, said lower chamber portion being filled with water and particulate buoyancy material, e.g. polystyrene particles that are immersed in water, and said upper chamber portion being filled with water. The other chambers are filled with air and comprise air chambers provided at each longitudinal side of said mattress for stiffening its lateral edges, and adjustment chambers provided at the bottom of said mattress and provided with a common valve for adjustment of the pressure in said water bed mattress. Furthermore, central chambers are provided at the bottom of said mattress between said adjustment chambers. Said air chambers are always filled with air to absorb loads in case of sagging when the top surface of said water bed mattress is subjected to an excessive punctiform load on the upper surface of said water bed mattress. The intermediate bottom is secured to the bottom portion of said mattress and is arched upwards in breadth as well as in the longitudinal direction in an un-loaded state. This results in a water bed mattress having a small volume of water, since this is partly replaced by said air chambers and the particulate buoyancy material.