The organic metal &agr;‐[bis(ethylenedithio) tetrathiofulvalene]2I3shows a large third‐order optical nonlinearity around 650 nm, as observed by forward degenerate four‐wave mixing. Nonlinear susceptibilities of ‖&khgr;(3)‖ ∼5×10−8esu are observed along the direction of maximum conductivity. The frequency dispersion of the nonlinearity points to a plasma effect due to the conduction electrons.