The capabilities of frequency-selective surface (FSS) as free-space filters for wireless communications in the 300 GHz band are here demonstrated experimentally. The presence of a filter plays a key role to protect wireless communications from out-of-band interference without resorting to computation intensive forward error correction schemes. An FSS filter is suitably designed to exhibit an almost linear phase and a low insertion loss over a 20 GHz bandwidth centered around 270 GHz. A method based on both full-wave simulations and numerical techniques is developed to predict the bit-error rate (BER) performance of a filter-based wireless communication when an on-off keying modulated signal is sent at 14 Gb/s. Results are corroborated through measurements and confirm that the filters marginally affect the BER within the operating bandwidth while it is capable of strongly rejecting out-of-band interferences. In future works, the performance of more elaborated filters and modulations schemes will be investigated in detail.