The design and fabrication of a wideband millimetre‐wave 0‐dB coupler is proposed in this paper using gap waveguide technology for low‐loss and high‐power applications in 30‐GHz frequency band. To overcome the fabrication challenges in millimetre‐wave frequencies, the gap waveguide technique is utilised. Two gap waveguide‐based coaxial‐ and waveguide‐fed 0‐dB couplers are designed with broadband performance, high return loss, acceptable coupling flatness and high isolation. For verifying the performance of the proposed structures, a prototype of the waveguide‐fed 0‐dB coupler is manufactured and measured. The measurement results show that the return and insertion losses and the isolation of the fabricated 0‐dB coupler is better than 18 dB, 0.5 and 18 dB, respectively, in the specified frequency range from 26.2 to 34 GHz. Moreover, the breakdown power level of the proposed millimetre‐wave structures is in kW orders to satisfy the high‐power requirements.