<p>Produced water, a major by-product of oil and gas extraction, presents serious environmental challenges due to its high turbidity, oil and grease content, and potential for contaminant bioaccumulation. This study reports the synthesis, optimization, and performance evaluation of a novel organic–inorganic composite coagulant. Polyaluminium ferric chloride (PAFC) was synthesized, and then composite coagulant was prepared by grafting high-viscosity polyamine (HVP) onto PAFC. The coagulant composition was optimized at an Al/Fe ratio of 7, basicity (OH/(Al + Fe)) of 2.0, and 10% polyamine content based on its efficacy assessed in both synthetic and real produced water and compared against conventional coagulants, including polyaluminium chloride (PAC) and ferric chloride (FeCl₃).The PAFC–polyamine composite coagulant exhibited a synergistic mechanism, combining the high charge density of PAFC for charge neutralization with polymeric chains of polyamine for bridging and sweep flocculation. This resulted in superior performance, achieving up to 95% turbidity reduction and 90% oil and grease removal in real produced water at lower dosages than PAC and FeCl₃. Additionally, the hybrid coagulant generated approximately 55% less sludge than conventional metal salts, demonstrating enhanced operational sustainability. Speciation analysis revealed that optimizing the Al/Fe ratio and basicity shifted aluminum and iron species toward highly effective polymeric forms, further improving coagulation efficiency. Overall, the study demonstrates that the PAFC–polyamine composite coagulant is a highly effective, low-waste, and environmentally sustainable solution for the treatment of oil and gas produced water, offering both improved contaminant removal and operational efficiency.</p>