AbstractHerein, a semianalytical model is presented for solitary wave diffraction by a surface-piercing concentric dual-arc thin wall. The two arc-shaped walls are porous and thin, and they are placed on a flat seafloor. As a key element, two imaginary closed cylindrical structures with two arc-shaped walls of different radii are introduced such that the entire computation domain can be partitioned into three subdomains, two bounded and one unbounded, based on common interfaces, within which the analytical solution is obtained using eigenfunction expression matching. Furthermore, a system of linear algebraic equations to determine the unknown coefficients is derived by satisfying the boundary and matching conditions. The numerical results obtained for the limiting cases are exactly the same as published results for a solid and porous cylindrical structure. Meanwhile, the effects of the wave incident angle, opening angle, annular spacing, and porous-effect parameter on wave loads and wave elevations are investigated. In addition, as a particular case, a concentric structure with an arc-shaped porous exterior wall is investigated as an alternative to the two-cylinder structure in practical engineering. It is discovered that the arc-shaped exterior wall can effectively reduce the wave force and wave height around the interior cylinder, compared with a single cylinder. By contrast, the wave forces and wave runup on the interior cylinder are similar to those of the interior cylinder of a concentric two-cylinder system.