A novel substrate integrated waveguide (SIW) cavity-backed selftriplexing slot antenna, consisting of a hybrid SIW cavity resonator is proposed for tri-frequency operation. The hybrid cavity resonator is an amalgamation of a half-mode circular and half-mode rectangular SIW cavity resonator. A radiating slot is etched on its top surface, which produces three distinct resonant frequencies (5.57, 7.17 and 7.65 GHz) when it is excited by three microstrip lines. The lower band around 5.57 GHz is achieved due to a half-mode circular cavity resonator, when the corresponding microstrip line is excited. On the other hand, remaining two bands around 7.17 and 7.65 GHz are achieved due to half-mode rectangular cavity resonator when excited by the corresponding microstrip lines. The experimental isolation level between any two feed lines is better than 20 dB, which helps to introduce the self-triplexing phenomenon. Moreover, the proposed design is realised on a single-layered copper-laminated dielectric substrate, maintaining planar configuration. © The Institution of Engineering and Technology 2018.