This paper reports the colorimetric sensing of Fe3+ ions in water using magnesium oxide (MgO) nanoparticles where a novel and green method to synthesize MgO nanoparticles (NPs) using Syzygium aromaticum extract (clove extract) as reducing agent has been described. The synthesized MgO NPs were characterised by various techniques i.e. UV-Vis, Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), and Energy dispersive X-ray analysis (EDX). The MgO NPs were used as colorimetric probe for the sensitive and selective sensing of Fe3+ ions in water using UV-Vis spectrophotometry. Sensing efficiency of MgO NPs towards Fe3+ was investigated at pH range of 3-10. The best selectivity and sensitivity was obtained at pH 3. Limit of detection was calculated using 3 sigma method and it was found to be 23 mu M. Further, application of MgO NPs in practical sensing of Fe3+ in real water samples was also successfully investigated. (C) 2018 Published by Elsevier B.V.