This article presents a miniaturized self-diplexing slot antenna utilizing a substrate-integrated rectangular waveguide (SIRW) for millimeter-wave applications. The proposed antenna is built using a rectangular cavity, a slot on the top conductor, and two microstrip-feed lines. The rectangular cavity is realized for operation at \(TE_{110}\) mode (26.5 GHz), which is perturbed for radiation at two distinct frequency bands by loading a slot on the top conductor. Two microstrip-feed lines are employed for excitation of the slot, which generates two operating bands. For better understanding of radiation phenomena, the electric field distribution, equivalent circuit model, and parametric analysis are provided. To further validate the proposed design, a millimeter-wave self-diplexing antenna operating at 22 and 28 GHz is built and full-wave simulated. The proposed antenna occupies a smaller footprint area of 0.396 \(\lambda _g^2\) with an isolation greater than 20 dB. Additionally, the suggested antenna offers low cross-polarization level better than −30 dB. The realized gain and efficiency of the proposed antenna at lower (upper) frequency bands are 5.5 dBi (5.4 dBi) and 90.6% (94%), respectively.