Klystrons are vacuum electronic devices widely used for microwave amplification up to the millimetre-wave region. The opportunity to push the operating frequency of conventional klystrons above 30–40 GHz is limited by the fabrication difficulties of the cavities that have very small dimensions. A high order mode micro re-entrant square cavity was purposely devised to be compatible with the available micro fabrication processes and represents a viable solution for the design of millimetre-wave klystrons. In this study, a coupler to correctly excite the higher order mode of the square re-entrant cavities for W-band operation is proposed. The design of the proposed novel coupler structure has been validated by realisation and measurement of a scaled X-band prototype. The post-realisation tuning methods adopted to improve the matching of the high order mode are also presented.