In this letter, a single‐layer circularly polarized substrate integrated waveguide (SIW) square ring‐slot antenna operating in the X‐band is proposed.The circularly polarized wave is generated by a combination of a shortened square ring‐slot etched in the SIW top wall and a shorting via placed between the area bounded by the ring and the SIW bottom wall. Experimental results prove that the antenna with a coax‐to‐SIW transition radiates a right‐hand circularly polarized (RHCP) wave and it achieves the impedance bandwidth of 10.3% for the reflection coefficient less than –10 dB, the axial ratio (AR) bandwidth of 2.6% for AR less than 3 dB, and the RHCP gain 8.0 dBi. The proposed antenna has the advantage of conventional planar structures including low profile, light weight, and easy low cost fabrication. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2590–2594, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27113