Abstract: A portable mattress stiffener forming a supplemental support adapted to be placed between the mattress and underlying box springs, wherein a plurality of elongated rigid slats of relatively thin, generally rectangular cross-sectional configuration are arranged in planiform side-by-side parallel abutting relation in use position to form a rectangular panel. L-shaped hinge formations integrally formed along one side wall of each slat protrude beyond the face of the side wall and a rod portion of the opposite side wall interfits in a recess defined by the L-shaped protruding hinge formation to form hinges between the slats at their lower edges permitting the slats to be rolled up into a roll-like bundle for transport, or to be unrolled into a predetermined substantially planiform relationship with the hinge axes of the interfitting hinge formations located adjacent the lower edges of the side walls and the side walls abutting each other.