The transition of a Mott insulator to metal, the Mott transition, can occur via carrier doping by elemental substitution(1), and by photoirradiation, as observed in transition-metal compounds(2-4) and in organic materials(5). Here, we show that the application of a strong electric field can induce a Mott transition by a new pathway, namely through impulsive dielectric breakdown. Irradiation of a terahertz electric-field pulse on an ET-based compound, kappa-(ET)(2)Cu[N(CN)(2)] Br (ET:bis(ethylenedithio) tetrathiafulvalene) 6, collapses the original Mott gap of similar to 30 meV with a similar to 0.1 ps time constant after doublon-holon pair productions by quantum tunnelling processes, as indicated by the nonlinear increase of Drude-like low-energy spectral weights. Additionally, we demonstrate metallization using this method is faster than that by a femtosecond laser-pulse irradiation and that the transition dynamics are more electronic and coherent. Thus, strong terahertz-pulse irradiation is an effective approach to achieve a purely electronic Mott transition, enhancing the understanding of its quantum nature.