The third-order optical nonlinearities of dimethyl sulfoxide (DMSO) are investigated using the optical Kerr effect (OKE) and Z-scan techniques with femtosecond pulses. We are able to fit the OKE signal theoretically by convolving the autocorrelation of the incident pulse intensity profile with the impulse responses of the samples, illustrating the instantaneous nonlinear response of DMSO. We verify that the purely electronic cloud distortion causes the observed signal for DMSO. The Z-scan technique is used to estimate the third-order susceptibility of DMSO, which is about a quarter of that for CS2 and mainly comes from the nonlinear refraction with an intensity of less than 140 GW/cm2.