This study explores the biosynthesis of iron oxide nanoparticles (IONPs) using Bacillus circulans bacterial supernatant as an inexpensive and sustainable alternative to traditional synthesis approaches. The resulting nanoparticles were found to have a uniform size distribution and a spherical shape and agglomerated to form clusters as observed through scanning electron microscopy (SEM). The average diameter of IONPs determined using SEM was 18.37 nm. Characterization using energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) confirmed the synthesis of maghemite (γ-Fe2O3) nanoparticles, with a crystallite size of 13.84 nm. Fourier-transform infrared spectroscopy (FT-IR) revealed the surface functional groups of IONPs. Additionally, the IONPs were evaluated for antioxidant activity using the DPPH and ABTS assays, displaying 39.44% inhibition of ABTS radical cations and 35.44% DPPH scavenging. Calcination of IONPs for two hours at 300°C resulted in the conversion to the hematite (α-Fe2O3) phase, with a crystallite size of 23.18 nm, and slightly lower antioxidant activity, 35.04% inhibition of ABTS radical cations, and 26.5% DPPH scavenging.