We report on the observation of terahertz (THz) radiation induced band-to-band impact ionization in HgTe quantum well (QW) structures of critical thickness, which are characterized by a nearly linear energy dispersion. The THz electric field drives the carriers initializing electron-hole pair generation. The carrier multiplication is observed for photon energies less than the energy gap under the condition that the product of the radiation angular frequency ω and momentum relaxation time τ l larger than unity. In this case, the charge carriers acquire high energies solely because of collisions in the presence of a high-frequency electric field. The developed microscopic theory shows that the probability of the light-induced impact ionization is proportional to exp ( − E 0 2 / E 2 ) , with the radiation electric field amplitude E and the characteristic field parameter E 0 . As observed in experiment, it exhibits a strong frequency dependence for ω τ ≫ 1 characterized by the characteristic field E 0 linearly increasing with the radiation frequency ω .