In this work, we design and experimentally demonstrate a novel terahertz (THz) 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 two-dimensional 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 out-of-band rejection, as theoretically expected. This kind of component provides a cost-effective, functional solution for narrowband filtering in emerging THz devices and systems with possible applications in wireless telecommunications.