A novel simple decoupling technique is proposed to suppress the mutual coupling of a millimeter-wave multiple-input-multiple-output dielectric resonator antenna (DRA). The design to be decoupled consists of two microstrip-line fed rectangular DRAs placed side-by-side on an RT-Duroid substrate and operates near 26 GHz. First, the DRAs are placed at an appropriate separation distance to avoid any close coupling. Second, to reduce coupling further, a set of square ring cell loaded slot is employed on the ground plane in between the DRAs. These two arrangements together, potentially reduce the mutual coupling between the DR elements and substantially improves the in-band isolation from 14 to 23 dB (min)/34 dB (max) between them. The design is very simple, compact, and easy to fabricate. The model is prototyped and characterized to demonstrate its feasibility for operation. This can be a suitable technique for fifth-generation compact handheld microwave devices.