The linearized theory of water waves is used to examine the diffraction of incident regular waves by a slotted dual cylindrical caisson breakwater. The breakwater consists of a vertically slotted outer cylinder circumscribing an impermeable inner cylinder. Under the assumption that the wavelength is much greater than its thickness, each panel is replaced by a thin structure and the permeability is modeled by suitable boundary conditions applied on its surface. The mixed boundary‐value problem for the outer cylinder is transformed to dual series relations and the least‐square method is applied to get the forces on the structure and associated diffraction field. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)