The iron oxide nanoparticles (IONPs) prepared by green synthesis method using Syzigium cumini leaf extract was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The XRD confirmed the crystalline structure of green synthesized NPs measuring around 33 nm while SEM revealed its nearly spherical shape. Rhizobium species recovered from greengram nodules, identified by 16s rRNA gene sequencing as Rhizobium pusense produced 30% more exopolysaccharides (EPS) in basal medium treated with 1000 μg IONPs/ml. Compositional variation in EPS was observed by Fourier-transform infrared spectroscopy (FTIR). There was no reduction in rhizobial viability and no damage to bacterial membrane was observed under SEM and confocal laser scanning microscopy (CLSM), respectively. Effects of IONPs and R. pusense, used alone and in combination on the growth and development of greengram plants varied considerably. Plants grown with IONPs and R. pusence, used alone and in combination, showed a significant increase in seed germination rate, length and dry biomass of plant organs and seed components compared to controls. The IONPs in the presence of rhizobial strain further increased seed germination, plant growth, seed protein and pigments. Greater protein content (442 mg/g) was observed in seeds at 250 mg/kg of IONPs compared to control. Plants raised with mixture of IONPs plus R. pusense had maximum chlorophyll content (39.2 mg/g FW) while proline content decreased by 53% relative to controls. This study confirms that the green synthesis of IONPs from S. cumini leaf possess useful plant growth promoting effects and could be developed as a nano-biofertilizer for optimizing legume production.