This letter proposes a new decoupling approach to the closely spaced square patch antennas without extra space or additional layer by employing and placing the short-circuited half-wavelength (λ/2) microstrip line between adjacent antennas. The decoupling mechanism of a cascaded, series resonant, half-wavelength resonator is intuitively revealed by a simple equivalent circuit model. As numerically simulated and experimentally validated, the isolation at the center frequency can be remarkably enhanced from 7.5 to 21.3 dB over the entire passband with a close edge-to-edge spacing of 0.034λ0 and center-to-center spacing of 0.329λ0. As a result, the simulated and measured in-band envelope correlation coefficients (ECCs) achieved as low as 0.0016 and 0.01, respectively.