Abstract This article reports a simple, cost-effective, and eco-friendly biosynthesis of ZnO/Fe₃O₄ nanocomposites using Callistemon viminalis leaves’ water extract. For the first time, we used a green synthetic route via C. viminalis leaves’ extract to prepare ZnO/Fe₃O₄ nanocomposites (NCs) using zinc acetate and ferric chloride as precursor materials. Fourier transform infrared (FTIR) spectroscopic results revealed polyphenolic compounds mainly phenolic acids present in the plant extract acted as both reducing and stabilizing agents to synthesize ZnO/Fe₃O₄ NCs. Outcomes of XRD and X-ray photoelectron spectroscopy confirmed the formation of ZnO–Fe₃O₄ heterojunction in ZnO/Fe₃O₄ NCs, with crystallite sizes of 45, 35, and 60 nm, respectively, according to Debye–Scherrer’s formula. EDX confirmed Zn, Fe, and O in the ZnO/Fe₃O₄ nanocomposite. Scanning electron microscopy and transmission electron microscopy (TEM) analyses revealed the existence of both ZnO and Fe₃O₄ in the NCs with some agglomeration. The thermal stability of NCs was evaluated using thermogravimetric analysis (TGA) and differential thermal analysis (DTA) in a nitrogen atmosphere. In addition, as-prepared ZnO/Fe₃O₄ NCs along with biosynthesized ZnO and Fe₃O₄ (prepared by C. viminalis extract) nanoparticles were examined for photodegradation of methylene blue under visible light irradiation for 150 min. The result reveals that the photodegradation efficiency of ZnO/Fe₃O₄ NCs (99.09%) was higher compared to that of monometallic ZnO (84.7%) and Fe₃O₄ (37.1%) nanoparticles.