Abstract—The overwhelming demand for high-data-rate applications and low latency, which are prerequisites for multimedia content, are propelling technological advancements toward 5G communication networks. To acquire the intended 5G requirements and further encapsulate many application areas, a number of technologies has been implemented including millimeter waves and multiple input multiple output (MIMO) systems. One of the primary limitations in creating small MIMO antennas is the presence of inter-element mutual coupling. To cope with the non-desired mutual coupling, this work embodies a defected ground structure (DGS) MIMO antenna operating in a millimeter-wave 5G band. The proposed MIMO array antenna comprises four ports with eight identical patches. It has a total surface area of 50×60 mm², and is printed on FR4 epoxy substrates with a 4.4 dielectric constant. The bandwidth of the presented structure is extended thanks to the incorporation of slots. The simulation results demonstrate a wide bandwidth covering from 26.9 to 29.3 GHz, and that owing to the DGS, the mutual coupling is alleviated. Subsequently, a high level of isolation (greater than -27 dB), and an ultimate peak gain of 5.525 dBi are reached over the resonance bandwidth. Moreover, investigation of the MIMO diversity performance shows the following parameters: the envelope correlation coefficient (ECC) < 0.0003, diversity gain (DG) > 9.999 dB, and the total active reflection coefficient (TARC) < -10 dB in the operating band. Additionally, the antenna is found to cover the band allocated to 5G in both USA (27.5-28.35 GHz) and Japan (27.5-28.28 GHz). Based on the obtained results, the proposed MIMO array antenna is useful for application in both 5G band handsets and future IoT applications.