We report the first measurement of the third‐order nonlinear susceptibility &khgr;(3)in layered semiconductor clusters exhibiting pronounced quantum size effects at room temperature. BiI3clusters prepared in colloidal form in acetonitrile had a thickness of &bartil;7 A˚ and lateral dimensions between 60 and 90 A˚. Using degenerate four‐wave mixing, we observed that the conjugate pulses from the small and the large gratings had comparable intensities, verifying the electronic origin of the nonlinearity. The nonlinear susceptibility was found to be 2.3×10−11esu for a colloid with a cluster volume fraction of 10−5.