This study introduces a novel, green-synthesized TiO2/CeZrO4 nanocomposite (NCs) as a highly efficient visible-light-driven photocatalyst for malachite green (MG) degradation. The TiO2/CeZrO4 NCs were synthesized using a sustainable method employing Momordica charantia L. (bitter melon) leaf extract, a novel approach leveraging its high alkaloid content as a natural weak base and capping agent. Comprehensive characterization confirmed the successful formation of the composite structure. UV-DRS revealed a significantly reduced band gap for the TiO2/CeZrO4 NCs (2.71 eV) compared with pristine TiO2 NPs, thereby extending their light absorption into the visible spectrum. Direct visual evidence of the heterojunction formation was obtained from HR-TEM, which showed distinct lattice fringes (0.24 nm for TiO2 (004) and 0.30 nm for CeZrO4 (222)) in intimate contact within the nanocomposite particles, corroborated by SAED patterns. XPS analysis further confirmed the elemental composition and chemical states, revealing crucial insights into interfacial electronic interactions, including the presence of oxygen vacancies and enhanced Ce3+/Ce4+ redox cycling, indicative of strong heterojunction formation. Photoluminescence spectroscopy demonstrated a substantial quenching of emission intensity in the TiO2/CeZrO4 NCs compared to TiO2 NPs, confirming highly efficient charge carrier separation and suppressed electron-hole recombination, a hallmark of the S-scheme mechanism. Under visible light, the TiO2/CeZrO4 NCs exhibited remarkable photocatalytic activity, achieving 92.30% degradation of malachite green within 120 min with an optimum catalyst mass of 5 mg. This far surpasses the degradation efficiencies of individual TiO2 NPs (23.26%) and CeZrO4 NPs (82.17%). The enhanced performance is attributed to the synergistic effect of the S-scheme heterojunction, which ensures prolonged charge separation and the efficient generation of reactive oxygen species (•O2− and •OH). The photodegradation kinetics followed a pseudo-first-order model, with the highest rate constant for the nanocomposite (9.4 ×10-3 min−1). This research highlights a significant advancement in green-synthesized, high-performance photocatalysts for environmental remediation. © 2025 Elsevier B.V.