A co-design of a wideband circularly polarized (CP) patch antenna is proposed in this paper. The patch antenna is seamlessly integrated with two bandpass filter networks consisting of one and two λ/2 resonators, respectively, which are designed to excite two orthogonal modes with equal magnitude and quadrature phase over a widened bandwidth. In co-design of this filtering antenna, the patch antenna is equivalently treated as the last-stage resonator and the output port of the filter network. In analysis of its far field, axial ratio (AR) of CP wave is mapped to its magnitude and phase responses. By simultaneously synthesizing the magnitude ratio and phase difference of the two filter networks' transmission coefficients to vary around 1° and 90°, respectively, wideband AR response with three minima is realized. Based on this design method and the theoretically derived results, a CP patch antenna prototype with a height of 0.028 λ is fabricated and measured. It is experimentally demonstrated that around 8.8%, bandwidth can be achieved with VSWR < 2, AR < 3 dB, and 3 dB gain variation.