This study introduces an innovative MIMO antenna tailored for 5G millimeter-wave applications. By integrating a rectangular closed loop (RCL) and split-ring resonators (SRRs) with a monopole structure, the design achieves notable enhancement in impedance performance around the 26.5 GHz frequency. The MIMO configuration comprises four radiating elements positioned on a common PCB with a space-efficient footprint of 30 × 30mm2. The developed four-port antenna system offers a wide impedance bandwidth of 2 GHz (25.7-27.7 GHz) centred at 26.5 GHz, achieving a total efficiency of 85-95% over the operating band. Additionally, the antenna exhibits envelope correlation coefficient (ECC) values within acceptable limits, ensuring excellent isolation between the ports. The antenna achieves an average gain of 7 dBi, confirming its effectiveness for deployment in millimeter-wave 5G New Radio (NR) bands n257, n258, and n261.