In recent years, pressure retarded osmosis (PRO) has drawn considerable attention due to its feasibility of harvesting clean and sustainable osmotic power through a membrane process. When municipal wastewater is fed into this process, fouling in the porous substrate of the PRO membrane is the major concern since it can hinder the membrane performance tremendously and lower the power generation. In this work, sulfonated hyperbranched polyglycerol (SHPG) polymers with a dendritic architecture have been molecularly designed and then grafted onto the surface of polydopamine (PDA) coated poly(ethersulfone) (PES) hollow fiber membranes. Comparing to the pristine PES membranes, the SHPG modified PRO membranes show significantly improved resistance to protein adhesion and bacterial attachment due to the high wettability of their ionic polymer brushes. In PRO tests under different hydraulic pressures, the SHPG-grafted membranes show a lower flux reduction and a higher flux recovery rate of 94% vs. 87% in comparison with the pristine PES membranes. Therefore, the osmotic power generation can be significantly sustained by modifying the PRO membranes with SHPG polymers. This work may provide a versatile approach and useful insights for the design of membranes with antifouling properties in the molecular level.