© 2020 Author(s). We report the experimental observation of the refraction and reflection of propagating magnetostatic spin waves crossing a 90° domain wall (DW). Time-resolved magneto-optical imaging was used to observe the propagation dynamics of magnetostatic spin waves. Due to the magnetization rotation across such a DW, the dispersion relation of magnetostatic spin waves rotates by 90°. This results in a change in the propagation dynamics of spin waves on both sides of the DW. We observed the refraction and reflection of magnetostatic spin waves at the DW and reveal their characteristics that include negative refraction. The incident-angle dependence of the refraction angle is explained by the wavenumber conservation along the DW, quite similar to the case of Snell's law for light.