In this study, a hydrophobic porous organic polymer (PP) synthesized via Friedel-Crafts alkylation of dichloro-p-xylene, was functionalized with hydrophilic sulfonate functional group to form PP-SO3H, prior to incorporation into the polyamide layer of a hollow fiber pressure retarded osmosis (PRO) TFN membrane. Sulfonate functionalization of the PP material improved the compatibility of the nanomaterial with the aqueous amine precursor during the in situ interfacial polymerization, leading to a decrease in aggregation of the nanoparticles. The effect of nanomaterial loading on the membranes' performance was also elucidated. PP-SO3H incorporation resulted in significant improvement of surface hydrophilicity, which along with improved porosity, facilitated enhanced water transport across the membrane and maintained an acceptable salt rejection of the selective layer. The best-performing membrane with PP-SO3H loading of 0.002 wt% showed a 46.3 L m−2 h−1 water flux and power density of 14.6 W m−2, which are both the highest values recorded. This study suggests that the functionality and chemical properties of the nanomaterial in a TFN membrane is essential in improving altogether the osmotic performance and salinity gradient power harvesting capability during PRO.