We demonstrate theoretically and experimentally how changes of a terahertz (THz) beam induced by the sample affect the accuracy of the determination of THz dielectric properties in THz time-domain transmission spectroscopy (TDTS). We apply a Gaussian beam and the ABCD matrix formalism to describe the propagation of the THz beam in a focused beam setup. The insertion of the sample induces a focus displacement which is absent in the reference measurement without a sample. We show how the focus displacement can be corrected. The THz optical properties after focus displacement correction reported in this Letter are in quantitative agreement with those obtained using collimated beam THz–TDTS in previous work.