The invention pertains to a waterbed with at least one water-filled mattress, which forms a substantially flat bed surface when in an unloaded state and which is held inside a frame consisting of beams of foamed plastics. On the inside of the waterbed, at least one beam is convexly curved, proceeding upward in an arc from a carrier plate and continuing as a flat surface at the top, underneath the surface of the bed. The mattress, on the side facing the beam, has a concave recess conforming to the arc of the beam and has an extension of rectangular cross section, which covers the top surface of the beam, the extension preferably extending all the way to, and being flush with, the top outside edge of the beam.