In a THz pump - optical probe experiment we demonstrate an instantaneous electro-absorption effect in InGaAs/GaAs quantum dots, induced by the electric field of a single-cycle THz pulse with 3 THz bandwidth and with free-space peak electric field reaching 220 kV/cm. The transient modulation of QD ground state optical absorption at 1040 nm coherently follows the evolution of the absolute value of THz electric field. The optical modulation signal was found to be as short as 460 fs at FWHM, and retained the 3 THz bandwidth of the THz pulse. Optical absorption modulation in QDs by the THz field is dominated by absorption quenching effect due to the electron- and hole wavefunction separation in THz field, rather than by the Stark shift away from the optical probe.