Proton nuclear magnetic resonance (NMR) relaxometry is proposed to determine the iron redox ratio of slag samples. It depends on the paramagnetic effect on bulk water relaxation times. Experiments have been carried out to calibrate the relationship between the concentration of Fe3+ and Fe2+ ions and the measured relaxation times on prepared standard samples. It is shown that Fe2+ ions have very different effects on the relaxation times than Fe3+ ions. For the solid slag samples, digestion in an acid was used to produce solutions with a pH of 1.0. Hydrogen peroxide (H2O2) was then added to the digested samples to oxidize Fe2+ ions to Fe3+ ions. The concentration of Fe2+ and Fe3+ ions in solution can be calculated by measuring the relaxation times before and after oxidation. The Fe3+/∑Fe ratio of slag samples was also investigated by wet chemistry and 57Fe Mössbauer spectroscopy and calculated by FactSage at the equilibrium state. The results obtained from different methods are consistent, indicating that this NMR relaxometry method is reliable.