In this paper, ZnS nanoparticles deposited on montmorillonite (ZnS-MMT) were prepared by a facile method at room temperature and characterized by powder X-ray diffraction (XRD), Energy-dispersive X-ray Detector (EDX) and transmission electron microscope (TEM), respectively. Significantly, the as-prepared ZnS-MMT nanocomposites have been proven to possess intrinsic peroxidase-like activity that can rapidly catalyze the reaction of peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H2O2 and produce a blue color product in less than 30 seconds, which provides a sensitive colorimetric sensor to detect H2O2. Due to the synergistic effects between montmorillonite and ZnS nanoparticles, the obtained ZnS-MMT nanocomposites exhibit higher catalytic activity than that of MMT or ZnS alone. The catalytic behaviors of the ZnS-MMT nanocomposites showed a typical Michaelis–Menten kinetics. The catalytic activity and the catalytic mechanism were investigated using the procedures of steady-state kinetics and hydroxyl radical detection. ESR data revealed that the peroxidase-like activity of ZnS-MMT originated from the generation of radical dotOH radicals.