•The first report of fluorescence quenching of MoS2 QDs by Fe3+ ion, to our best knowledge.•The fluorescent MoS2 QDs–Fe3+ sensor operates conveniently and effectively in extremely low concentrations.•The fluorescent MoS2 QDs–Fe3+ sensor is label free and has good selectivity. MoS2 quantum dots were successfully prepared by a one-step hydrothermal exfoliation procedure. The prepared MoS2 quantum dots are monodispersed and have a circular shape, with an average diameter of 4.67 nm. Excited by UV-light, MoS2 quantum dots emit blue-purple fluorescence, and the fluorescence intensity is suppressed with the addition of Fe3+ ions. On this basis a novel fluorescent MoS2 quantum dots–Fe3+ label-free detection system is proposed. The decay curves of fluorescence show that the fluorescence quenching follows a static process. The binding stoichiometry ratio of Fe3+ ions toward MoS2 quantum dots is recognized as 1:1, based on the Benesi-Hildebrand and Job’s plot methods. For the concentration range of 0–50 μmol/L, the Fe3+ and fluorescence intensity are linearly related to 99.39%. The detection limit reaches 1 μmol/L. Several common ions were also examined that have little cross-influence on the fluorescence intensity of the MoS2 quantum dots. The fluorescent MoS2 quantum dots–Fe3+ probe has good performances and provides a novel, convenient and effective approach for the detection of Fe3+ of extremely low concentrations.