In this study, a wideband, low-loss waveguide power combiner is designed and manufactured using a waveguide-to-microstrip signal transition feed and resistive septum, which can be applied to waveguides in the W-band. The waveguide power combiner has a T-junction structure and is implemented by locating the transition feed at a length of λ/4, where the E-field is maximized, and combining the resistive septum manufactured by the NiCr thin film process. The performance is verified using a passive integrated circuit IC in the form of a microstrip line matched with 50 Ω to the manufactured combiner, and the combined characteristics according to the presence or absence of a resistive septum are confirmed by mounting a power amplifier PA. The isolation of the combiner with the resistive septum is −15 dB compared with the 9 dB without a resistive septum, which improves the simulation results. The power combining module manufactured using the passive IC exhibits a low insertion loss of approximately 0.75 dB (@ 78 GHz) after the output stage with the transition structure and including the resistive septum structure. As a result of constructing a power combining module with a waveguide structure using an MMIC with an amplification gain of approximately 20 dB and an output performance of 26 dBm, in the absence of a resistive septum, a low-power-combining characteristic of approximately 8 dB gain and 14 dBm is achieved. When a resistive septum is applied, it exhibits a stable amplification gain of approximately 18 dB and output performance of 28.4 dBm (2.4 dB combined gain).