In this Letter, a method for the broadband measurement of complex permittivity using a quasi-optical technique is presented. After calibration of the quasi-optical system, the material under test is inserted into the system and S-parameters are measured from 220 to 330 GHz using a vector network analyser. An extraction method is then applied to calculate the complex permittivity. The permittivity measurements are validated by comparing with a well-established extraction method and the comparison shows good agreement.