A circularly-polarized (CP) printed antenna is proposed for millimeter-wave mobile communications. The antenna has a defected ground structure (DGS), and it operates at 28 GHz. The geometries of the antenna on the upper surface and the DGS are symmetric about a [Formula: see text]-diagonal to produce perfect circular polarization. The antenna is fed through a three-stage microstrip line that acts as an impedance transformer for perfect matching of the antenna impedance to [Formula: see text] at [Formula: see text]. The development of the antenna design is described in detail, starting with a square patch that is subjected to progressive geometrical modifications in the design stages to reach the final design of the patch antenna. The simulation results show that the antenna has a reflection coefficient magnitude of less than [Formula: see text] at an axial ratio of less than 0.5 dB at 28 GHz. The technique used to generate the radiated circular polarized waves is demonstrated by investigating the surface current on the printed patch at the resonant frequency. The primary objective of the proposed design is not to produce a high-gain antenna, but rather to produce a low-profile planar antenna to serve as a building block for CP antenna systems such as multiple-input-multiple-output (MIMO) antennas, beam steering, and beamforming antenna arrays operating at 28 GHz. The designed antenna is fabricated and subjected to experimental evaluation. The numerical simulation results and the practical measurements came in good consent from the impedance matching and axial ratio (AR) of the radiated circular polarized wave perspectives. The input impedance is matched to the feeding line over a 1-GHz band, from 27.5 GHz to 28.5 GHz. The radiated wave is circularly polarized with AR less than 3 dB over about 0.4 GHz band starting from 27.8 GHz to 28.2 GHz. The peak gain of the designed antenna is 7 dBi and the radiation efficiency exceeds 90% all over the band of impedance matching. The performance of the antenna is compared with those of other antenna designs presented in some recent publications.