A Ka-band traveling-wave power divider/combiner, which is based on double ridge-waveguide couplers, is presented. The in-phase output, which is a challenge of the waveguide-based traveling-wave power divider, is achieved by optimizing the equivalent circuit of the proposed structure. The novel ridge-waveguide coupler has advantages of low loss, high power capability, and easy assembly. Finally, the proposed power divider/combiner is simulated, fabricated, and measured. A 15-dB measured return-loss bandwidth at the center frequency of 35 GHz is over 28%, a maximum transmission coefficients amplitude imbalance of ±1 dB is achieved, and the phase deviation is less than ± 12° from 32 to 39 GHz.