A high-harmonic traveling-wave tube (HHTWT) is designed, developed, and tested in this paper. This device is configured as a multiplier–amplifier with Q-band input signals and its third harmonic, i.e., D-band output power. The tested result demonstrates that 280 mW at 133.5 GHz is obtained as a result of 200 mW at 44.5-GHz input signal. The simulation and experiment validate that the high-harmonic radiation can be generated and amplified in the HHTWT. The approach described in this paper could find application in exploring high power, wideband tunable terahertz source. This paper is expanded from a conference paper on IVEC 2017. The detail design produce and tested result are added in this paper. The explanation of the simulation and tested results is also presented.