Abstract The performance of a spiral wound module operating under pressure retarded osmosis (PRO) is measured experimentally with a feed water back pressure. The module has a diameter of 0.1 m, a length of 1.02 m, and total membrane surface area of 7.45 m2 capsulated in a stainless-steel pressure vessel. The module is commercially designed for a reverse osmosis (RO) system however, a feed water outlet is added, and a back pressure is applied to adapt it for the PRO process. The experimental results of the spiral wound PRO module with a back pressure in the feed water side showed low power density values (about 0.35 W/m2) compared with those commonly found in the literature, demonstrating the geometric inefficiency of RO modules for the PRO process.