Flatfish bycatch is a concern in many demersal trawl fisheries around the world, especially for fisheries operating under discard-ban regulations. We introduce and assess the performance of FRESWIND (Flatfish Rigid EScape WINDows), a concept for a selection device that reduces flatfish bycatch in roundfish-directed fisheries. The new concept was tested for the first time in the Baltic cod-directed fishery, using a commercial twin trawler. The vessel was rigged with two trawls; one standard trawl gear and one incorporating the experimental FRESWIND. Comparison of the catches from both trawls exhibited up to ∼68% reduction in flatfish bycatch for the trawl with FRESWIND mounted. In addition, the catch of undersized cod was reduced by ∼30%, whereas losses of marketable cod were relatively minor (∼7%). Further simulations predicted that, in the commercial fishery, a reduction of more than 50% in flatfish bycatch could be achieved if FRESWIND were adopted. Given these promising results, FRESWIND may also provide a method that significantly reduces flatfish bycatch in other fisheries.