In this work, we design and experimentally demonstrate a novel terahertz filter exhibiting a flattened spectral response in the atmospheric transmission window around the central frequency of 300 GHz. The innovative concept behind this filter is the coupling of Fabry-Perot and guided-mode resonances. The latter arise from a 2D patch array patterned on an aluminum layer deposited on a low loss cyclo-olefin polymer. The filter experimental performance shows high transmittance in the flat-top band, with less than 3 dB losses, and high outof-band rejection, as theoretically expected. This kind of components provides a cost-effective functional solution for narrowband filtering in emerging THz devices and systems with possible applications in wireless telecommunications. http://dx. doi. org/10.1364/OL. 99.099999Recent years have seen a progressive filling of the so-called Terahertz (THz) gap thanks to the developing …