We report the facile and economic preparation of two-dimensional (2D) and 0D MoSe₂ nanostructures based on systematic and non-toxic top-down strategies. We demonstrate the intrinsic peroxidase-like activity of these MoSe₂ nanostructures. The catalytic processes begin with facilitated decomposition of H₂O₂ by using MoSe₂ nanostructures as peroxidase mimetics. In turn, a large amount of generated radicals oxidizes 3,3,5,5-tetramethylbenzidine (TMB) to produce a visible color reaction. The enzymatic kinetics of our MoSe₂ nanostructures complies with typical Michaelis–Menten theory. Catalytic kinetics study reveals a ping–pong mechanism. Moreover, the primary radical responsible for the oxidation of TMB was identified to be Ȯ₂⁻ by active species-trapping experiments. Based on the peroxidase mimicking property, we developed a new colorimetric method for H₂O₂ detection by using 2D and 0D MoSe₂ nanostructures. It is shown that the colorimetric sensing capability of our MoSe₂ catalysts is comparable to other 2D materials-based colorimetric platforms. For instance, the linear range of H₂O₂ detection is between 10 and 250 μM by using 2D functionalized MoSe₂ nanosheets as an artificial enzyme. Our work develops a systematic approach to use 2D materials to construct novel enzyme-free mimetic for a visual assay of H₂O₂, which has promising prospects in medical diagnosis and food security monitoring.