Two-dimensional (2D) transition metal dichalcogenides (TMDs) are drawing increasing interest due to their relatively high carrier mobilities, valley pseudospins, and gapped electronic structures, which all indicate interesting nonlinear optical properties of these 2D materials. However, such nonlinear optical properties are so far less investigated and their correlation with the electronic structure of the material is rarely probed. In this work, we have systematically investigated the spatial self-phase modulation (SSPM) of MoSe2 flakes in a suspension form, which is a coherent third-order nonlinear optical effect. The nonlinear susceptibility χ((3)) and its wavelength-dependence are measured, yielding a value of 1.1 × 10(-9) e.s.u. (SI: 1.53 × 10(-17) m(2)/V(2)) at 532 nm laser excitation for effective one-layer MoSe2.