This paper presents a novel, compact eight-port circular MIMO antenna system designed for millimeter-wave (mmWave) 5G communication, offering a wide impedance bandwidth and high isolation. The proposed antenna array operates over a broad measured frequency range of 25–35 GHz, achieving a 10 GHz bandwidth through a unique integration of circular and rectangular slots in the ground plane. Fabricated on a low-loss Rogers RT5880 substrate (εr = 2.2, thickness = 0.8 mm, tanδ = 0.0009), the design demonstrates excellent performance without requiring additional decoupling structures. The antenna achieves high isolation greater than 28 dB and a peak gain of 9.65 dB at 26 GHz and 28 GHz, enabling effective operation in high-attenuation mmWave environments. Compared to prior art, the presented system supports more antenna elements within a compact footprint while maintaining low Envelope Correlation Coefficient (ECC < 0.05) and high diversity gain, making it ideal for enhanced MIMO performance. Comprehensive analysis of S-parameters, radiation patterns, surface currents, and efficiency validates its suitability for next-generation 5G mmWave applications. The combination of compact geometry, high port count, wideband coverage, and exceptional isolation constitutes the core novelty of this work.