Forward osmosis (FO) is a promising technology for treating oily wastewater thanks to its high rejection and high water recovery. However, the performance of an FO membrane can be severely affected by the fouling of emulsified oil onto its rejection layer. In this work, we report a novel thin film composite (TFC) FO membrane with enhanced fouling resistance to emulsified oil. Specifically, we performed in situ surface grafting of its nascent polyamide active layer using amine-terminated sulfonated poly(arylene ether sulfone), i.e., NH2-BPSH100, over a range of molecular weights (2, 10 and 18 kg/mol). The results of multi-cycle fouling experiments using a feed solution of 40,000 ppm soybean oil/water emulsion showed optimum antifouling performance for the membrane modified with 10 kg/mol of NH2-BPSH100 (10k-g-TFC). The improved antifouling performance of this membrane can be attributed to its superhydrophilic and underwater superoleophobic properties. In the current study, the permeability of 10k-g-TFC membrane was easily recovered by simple hydraulic flushing. Even without any membrane cleaning, its flux maintained as high as 69.8% of its initial value at 80% water recovery, in direct contrast to 11.0% for the control membrane without surface modification. The current study has important implications for anti-fouling surface modification of FO membranes.