We obtained a new cation radical salt of bis(ethylenedithio) tetrathiafulvalene (ET) with kappa-type packing motif, kappa-(ET)(2)Ag(CN)[N(CN)(2)], by electrooxidation. In the salt, each ET layer is separated by polymeric {Ag(CN)[N(CN)(2)](-)}(infinity) layers, which are composed of Ag(I) ions trigonally coordinated to form an infinite zigzag chain of -NC-Ag-NC-Ag with pendant dicyanamides. The salt undergoes a superconducting transition with a midpoint critical temperature (T-c) of 6.6K and an onset T-c of 7.2 K, thereby indicating the highest T-c among ET superconductors formed with a polymeric Ag(I) complex anion.