Two spatial self-phase modulation (SSPM) patterns were observed in suspensions of B i 2 T e S e 2 nanosheets. Two mechanisms were found to produce SSPM with different occurrence times and power dependence. The Type I (narrow) rings are attributed to the coherent third-order nonlinear optical Kerr effect, which induces self-diffraction in 2D materials, and the Type II (wide) rings are assigned to the contribution of a thermal effect. The nonlinear refractive index of B i 2 T e S e 2 is found to be 2.30 × 10 − 5 c m 2 W − 1 at 700 nm. The findings described here provide an explanation for the formation of rings in 2D systems due to SSPM.