This paper introduces a two-port millimeter wave multiple input multiple output (MIMO) antenna designed with a compact area of 75 × 50 mm2. The proposed antenna comprises a cavity-backed substrate integrated waveguide (SIW), which results in improved performance. It provides a minimum isolation of 41 dB across the operating frequency band, ensuring maximum port decoupling. The envelope correlation coefficient (ECC) is well within the acceptable threshold over the operational bandwidth, supporting efficient MIMO operation. The proposed antenna resonates at 28 GHz and has a gain of 9.4193 dBi, making it a suitable option for millimeter wave communication systems in the n257 band, which is used for 5G applications.