<p>The continuous rise in global temperatures, driven by excessive energy consumption and harmful emissions, underscores the urgent requirement for sustainable as well as highly effective thermal management systems. Researches on various nanofluids (NFs) to boost heat transfer performance have attracted a lot of attention in recent years due to their excellent qualities. However, conventional nanoparticle synthesis often involves hazardous substances and harsh procedures, raising concerns about safety, ecological impact, and equipment degradation. As a greener alternative, biosynthesized nanoparticles (NPs) using plant extracts offer cost-effectiveness, reduced toxicity, and eco-friendly production pathways. This review focuses on recent advances in biogenic synthesis of copper oxide (CuO) NPs using extracts of plants as natural reducing and stabilizing agents and their applications in thermal systems. It highlights the structural, optical, and thermophysical advantages of biosynthesized CuO NPs such as enhanced stability, large surface area, and improved thermal conductivity which make them particularly suitable for thermal applications. Furthermore, factors influencing NP synthesis (e.g., precursor choice, plant extract concentration, pH, and reaction conditions) and their role in achieving colloidal stability are discussed. The review also examines latest empirical and statistical studies over the integration of bio-NFs into heat transfer systems, emphasizing their ability for endurable energy applications.</p> Graphical abstract <p>Graphical representation of the green synthesis of CuO nanoparticles using plant extract.</p> <p></p>