In this study, a millimetre-wave (MMW) antenna is presented for the fifth-generation (5G) wireless multiple-input multiple-output (MIMO) applications, in order to offer numerous advantages including compactness, planar geometry, high bandwidth, and high gain performance. The concept of defected ground structures has been deployed for the first time in MIMO antenna design at MMW spectrum to fulfil 5G requirements of high bandwidth with compactness and low design complexity. The top surface of antenna comprises of a coplanar waveguide-fed T-shaped radiating patch element, while the bottom part is designed to constitute a partial ground loaded with two iterations of symmetrical split-ring slots at an optimised distance. Experimental results depict a wide bandwidth of 25.1–37.5 GHz, as well as a peak gain of 10.6 dBi at 36 GHz. Moreover, numerically evaluated efficiency of >80% is observed over the entire intended bandwidth of operation. For the demonstration, four-element MIMO antenna array of the proposed antenna element is also fabricated and tested, in order to validate high isolation between the adjacent elements, which makes the proposed antenna a potential candidate to be integrated in cellular phones and base stations for 5G MIMO systems and services.