A novel decoupling method based on seamless integration of coupled-resonator filter for single-layer, size-miniaturized, broadbeam-radiating, closely spaced microstrip patch antenna (MPA) pair is presented. The method empowers MPA element with benefits of both significant size reduction and ultrawide beam expansion. Moreover, by packing two small MPA elements tightly, a seamlessly integrated coupled-resonator filter is naturally formed, providing a controllable coupling path with required amplitude and phase responses to cancel out the primary coupling path between the two patch radiators of the MPA pair. In particular, the miniaturization principle of the MPA element and the self-decoupling mechanism of the MPA pair are comprehensively revealed by equivalent circuit analysis. Measured results, in good agreement with simulated ones, demonstrate that an extremely low mutual coupling of -48.15 dB is obtained in the E-plane under an exceptionally small center-to-center spacing of 0.23 λ0 and edge-to-edge spacing of 0.003 λ0. Significantly, a wide half-power beamwidth (HPBW) of 142° is consistently obtained for individual elements. With distinct advantages, including thrifty compactness, broad beamwidth, and high isolation, the developed MPA pair is especially promising for space-limited terminals requiring wide wireless coverage, e.g., fiber-to-the-room (FTTR) applications.