In this paper, a balanced wideband filtering circularly polarized (CP) antenna designed using rectangular quasi-self-complementary structure is proposed. The antenna is fed by a differential microstrip line, and consists of a strip-loaded rectangular slot radiator and its complementary slot-loaded patch counterpart. The fields radiated by the two complementary parts of the antenna are expected to be orthogonal if properly excited. To achieve a CP radiation pattern, a 90° delay line section is embedded in one of the two feedlines to provide 90° phase difference between signals fed to the two antenna input ports. In addition, to suppress the wideband common-mode signals, two half-wavelength microstrip line sections that share part of their structures are connected between the two feedlines for producing two transmission zeros in the DM passband under CM operation. For the designed balanced wideband filtering CP antenna, measurement shows that the Sdd 11≤ -10 dB DM operating band covers a frequency range of 3.16-4.63 GHz. Very good CM suppression was observed with the measured CM reflection coefficient greater than -1 dB over most of the DM passband. The simulated 3-dB axial ratio band is 3.04-5.09 GHz.