In this communication, a cavity-backed slot antenna is proposed using substrate-integrated waveguide (SIW) technology for Wi-Fi/Industrial, Scientific, and Medical Radio band applications. This antenna comprises two SIW cavities, a rectangular and circular with a circular ring slot on the top of each cavity for radiation. The circular cavity is designed such that it is nested inside the rectangular cavity, and each one is excited by separate feedlines (microstrip and coaxial probe). By properly optimizing the antenna dimensions, it produces two frequency bands at 5.2 and 5.8 GHz with a small frequency ratio of 1.12. Finally, the design is fabricated and the results are experimentally verified. The experimental results show that the proposed antenna exhibits a -10 dB impedance bandwidth of 6% (5.04-5.35 GHz) in the lower frequency band and 3.4% (5.73-5.92 GHz) in the higher frequency band with maintaining the input port isolation better than 28 dB. Due to the SIW cavity-backing structure, the antenna shows unidirectional radiation patterns at both resonant frequencies of 5.2 and 5.8 GHz with the gain of 6.97 and 6.2 dBi, respectively. Moreover, the antenna possesses the advantages of lightweight, planar configuration, and ease of fabrication. © 1963-2012 IEEE.