Wideband linearly and circularly polarized (CP) filtering patch antennas with enhanced gain selectivity on a single-layer substrate are proposed. First, a linearly polarized (LP) filtering patch antenna is designed by connecting a λ/4 impedance transformer and a pair of dual-functional folded coupled-line resonators. The folded coupled-line resonator possesses very high susceptance-slope parameter as a parallel resonator on the one hand, and on the other hand it also provides two transmission zeros below and above the center frequency. The proposed feeding scheme and antenna topology not only achieve enhanced bandwidth and gain selectivity, but also have the merits of low parasitic radiation. Second, a CP antenna evolving from the conventional dual-fed design is developed for enhanced AR bandwidth (ARBW) by further utilizing the proposed dual-functional folded coupled-line resonator. Enhanced gain selectivity is achieved as well. Two minima in the AR response are generated to construct wide ARBW, which is almost the same as the impedance bandwidth. Finally, both the LP and CP filtering antennas are implemented on a single-layer substrate with a profile of 0.026λ. The theoretical performances are verified by the measured results.