The transition to green chemistry has prompted the development of eco-friendly methods for synthesizing functional nanomaterials. Herein, FeMnO₃ nanoparticles were synthesized using Nephelium lappaceum (rambutan) leaf extract as a natural hydrolyzing and stabilizing agent. Bioactive compounds such as flavonoids and saponins facilitated nanoparticle formation without toxic reagents. The material was characterized by FTIR, UV-Vis DRS, XRD, XPS, FE-SEM, and TEM. The FeMnO₃ nanoparticles were then employed as a heterogeneous catalyst in the Biginelli reaction to produce dihydropyrimidinones (DHPMs). Optimization of reaction parameters (5 mol% catalyst, methanol, 50 °C, 24 h) afforded up to 65 % yield, and the structure of the model product was confirmed by ¹H and ¹³C NMR. The catalyst exhibited good reusability over four cycles without significant loss of activity. This study demonstrates the potential of plant-mediated synthesis for generating sustainable nanocatalysts and advancing green multicomponent reactions. © 2025 The Authors