Abstract: A breakwater for attenuating the sea-swell effects of waves includes a bottom plate rigidly supporting and adapted to be immersed in a body of water. A generally upwardly extending solid front wall is formed at an upstream edge of the plate and an upper end of the front wall extends above the surface of the body of water. A generally upwardly extending rear wall is formed at a downstream edge of the bottom plate and has a plurality of regularly spaced orifices formed therein. A generally upwardly extending intermediate wall is formed between the front wall and the rear wall and also includes orifices, the front wall and the intermediate wall forming therebetween an energy dissipation basin. The front wall extends above the surface of the body of water so as to prevent the passage of a sea swell above the bottom plate as long as the amplitude of the swell is average.