Abstract: A modular box spring mattress includes two flexible plate units each of which consists of a plurality of plate subunits. The plate subunits of either of the plate units include a plurality of annular flanges which extend toward the adjoining plate unit. The annular flanges of one of the plate units respectively oppose the annular flanges of the adjoining plate unit so that the end turns of the compression springs are received within the annular flanges. Each of the annular flanges includes a plurality of flexible retaining arms extending obliquely and inwardly therefrom in a plane generally parallel to the plate subunits. The end portions of the retaining arms are spaced apart from the corresponding plate subunits so that the end turns of the compression springs can be respectively retained between the retaining arms and the plate subunits.