In this letter, a new method for the design of high-power low-pass harmonic filters in rectangular waveguide technology is proposed. Based on a 2-D arrangement of bandstop elements of sinusoidal profile, a filter with a wide spurious-free frequency response (up to the third harmonic) for the fundamental TE10 mode, as well as higher-order TEn0-mode suppression, is achieved. Compared to the classical waffle-iron solution, the novel proposal has a highly increased power-handling capability for the same frequency specifications.