<p>Forward osmosis (FO) is a membrane technology that has attracted significant attention recently for water purification and desalination. However, widespread approval of FO technology faces challenges, including membrane performance issues, notably internal concentration polarization (ICP), and the trade-off between water permeability and salt rejection. This study focuses on developing high-performance FO membranes by coating a polyelectrolyte interlayer onto a microfiltration (MF) substrate, characterized by its large pore size, which mitigates ICP and enhances water flux. However, the use of an MF substrate without modification presents challenges in forming a defect-free polyamide (PA) active layer due to the large and irregular surface pores. To solve this issue, a polyelectrolyte thin layer is coated on the MF substrate surface to control the synthesis of the PA layer. The interlayer is deposited using a layer-by-layer (LBL) assembly technique with sodium lignosulfonate (SLS) and polydimethyldiallylammonium chloride (PDMAC) polymers. The LbL number can control the surface hydrophilicity and pore size. The results demonstrate that coating the interlayer significantly enhances the water flux of FO membranes from 10.1 LMH in the control TFC to 20.5 LMH in TFC-LbL.3 (with 3 LbL number). The interlayer provides a smooth and more uniform interface for the polymerization reaction, resulting in a thin and uniform PA layer. This leads to increased water flux while maintaining high salt rejection. This claim is verified by the higher rejection of heavy metal ions in TFC-LbL.3 (98.3% for Cr³⁺ and 97.8% for Pb²⁺) in comparison to the TFC (95.4% for Cr³⁺ and 94.2% for Pb²⁺) membrane. The LBL assembly technique provides a cost-effective and environmentally sustainable method for fabricating high-performance FO membranes.</p>