A substrate-integrated waveguide (SIW) based quad-channel diplexer operating in 28-GHz and 39-GHz bands is proposed for the first time. To allow a greater degree of design flexibility, the quad-channel diplexer was developed with T-junction scheme and the power-current waveguide impedance was used for junction arm designs to fulfill the matching/open conditions required at self-/other channel frequencies. Moreover, a near-passband transmission zero is introduced by proper coupling control between higher/lower-order modes to improve in-band isolation. The fabricated prototype shows that, for the given (narrow) bandwidths of 4.3%, 1.7%, 2.4%, and 1.7%, the measured insertion/return losses are 3.5/15.3 dB at 26.7 GHz, 4.4/20.6 dB at 28.39 GHz, 5.5/28.4 dB at 37.31 GHz, and 6.5/20.1 dB at 38.44 GHz, respectively. The measured in-band isolation is greater than 35.7 dB, 47.8 dB, 34.7 dB, and 40 dB, while wideband isolation is greater than 32 dB over 20-45 GHz.