The forward osmosis (FO) process suffers from unfavorable internal concentration polarization (ICP) of the solute within the support layer of thin-film composite forward osmosis (TFC-FO) membranes. To lower the ICP effect, a support layer with low tortuosity, high porosity, and interconnected pores is necessary. In the present investigation, sodium bicarbonate has been presented as a simple pore-forming agent to decline the ICP within a poly(ethersulfone) substrate. In particular, the porous poly(ethersulfone) support layer was fabricated by embedding sodium bicarbonate into the casting solution to form CO2 gas bubbles in the substrate during phase inversion in an acidic nonsolvent. Experimental results revealed that the separation performance of the TFC-FO membranes significantly improved. The most water-permeable membrane was prepared in the acidic nonsolvent (TFC-SB.3) and it demonstrated a water flux of 26.6 LMH and a reverse salt flux of 3.6 gMH in the FO test. In addition, the TFC-SB.3 membrane showed an 85% increase in water permeability (2.13 LMH/bar) with negligible change in salt rejection (94.3%). Such observations were based on the increase of substrate porosity and the improved connectivity of the finger-like channels through in situ CO2 gas bubbling that alleviate the ICP phenomena. Therefore, the current study presents a simple, scalable method to design a high-performance TFC-FO membrane.