We present a facile methodology for ammonia (NH3) synthesis using a multiphase iron nanocatalyst (r-IOD (Fe@Fe3O4@Fe2O3)) under ambient conditions. Structural and physicochemical analyses of r-IOD were conducted using pXRD, Mössbauer spectroscopy, HRTEM, and XPS analyses, where r-IOD features a three-phase composite architecture, enabling room-temperature reduction of nitrate (NO3–) to NH3. The NH3 formation was confirmed by 15N isotope labeling and 1H NMR analysis and quantified by UV-Vis spectroscopic analysis. Postreaction UV-DRS and XRD of r-IOD were also analyzed, indicating Fe-redox participation in the process of NO3– reduction.