Abstract: The coupling between terahertz electromagnetic waves in free space and terahertz antennas/ probes is usually achieved through waveguides or silicon lenses. Waveguide-based coupling schemes can provide higher coupling efficiency than lens coupling schemes, whereas previously AlGaN/ GaN HEMT terahertz detectors/ mixers had the characteristic of high impedance and therefore generally used Si-lens coupling. In this paper, a waveguide-coupled AlGaN/ GaN HEMT terahertz direct detector is simulated and designed for 110 GHz applications. A waveguide-microstrip transition structure and the microstrip line impedance matching structure are applied to reduce the detector impedance to 275 Ω, which provides high responsivity while reducing the thermal noise voltage. After designing the structure of each part of the detector, the overall full-wave simulation, data extraction and postprocessing are performed, and finally the figure of merit of the detector, i. e. , the noise-equivalent power ( NEP), can be obtained. The results show that the NEP of the terahertz detector is about 22 pW/ Hz at 110 GHz, which is comparable to the sensitivity of the Si-lens coupled terahertz direct detector, where the gate length and gate width of the HEMT mixer in this detector are 1 μm and 150 μm, respectively. The simulation results suggest the NEP could be further reduced and the detector sensitivity can be improved by shrinking the gate length from 1 μm to 100 nm.