A simple decoupling technique based on a single-layer substrate is proposed for wideband closely-spaced patch antennas. By combining the coplanar decoupling structure (CDS) and defected ground structure (DGS), the strong coupling between wideband closely-spaced patch antennas can be significantly suppressed across the operating band without an additional substrate layer. To validate the feasibility of the proposed decoupling schemes, two decoupled wideband antenna prototypes with microstrip and probe gap-coupled feeding are designed, simulated, and measured at ISM and X band, respectively. Experimental results show that the H-plane decoupled prototype achieves over 15 dB isolation across a wide matched bandwidth of 18.4% with an edge-to-edge separation of 0.038 λ0 and the E-plane decoupled prototype achieves beyond 20 dB isolation across a wide matched bandwidth of 22.2% with an edge-to-edge separation of 0.033 λ0. Featuring a compact appearance, wideband operation, and strong-coupling suppression, the proposed decoupling technique is promising in wideband multiple-input and multiple-out (MIMO) applications.