This paper presents a novel self-octuplexing substrate-integrated waveguide (SIW) cavity-backed slot antenna for eight-band wireless communication services. This antenna employs eight semi-elliptical-shaped slots on an octagonal SIW cavity with different lengths to radiate at eight different frequencies. The eight radiating frequencies are 2.7, 3, 3.3, 3.6, 3.9, 4.2, 4.5, and 4.72 GHz, with impedance matching is less than −10 dB. Each radiator is backed by an eighth-mode SIW cavity and excited by a 50 Ω microstrip line. A designed prototype of the antenna is fabricated and tested. It has more than 34 dB measured isolation among any two ports. The maximum measured antenna gains are 4.4, 4.19, 4.11, 4.03, 4.19, 4.28, 5.2, and 5.44 dBi at the respective operating frequencies. It has a measured cross-polarization level of below −24.2 dB in the boresight direction and a front-to-back ratio of more than 10.4 dB in all the bands. All the slots can be tuned independently to radiate at the desired frequency band. This antenna can easily be integrated with other planar circuits due to an unperturbed ground plane.