A compact four-port stacked delta-shaped monopole antenna (SDSMA)-based MIMO array for Ka-band mm-wave communication is presented. The proposed design addresses the challenge of simultaneously achieving wide impedance bandwidth, high inter-port isolation, enhanced gain, and excellent diversity performance within a compact Ka-band MIMO antenna while avoiding the use of additional decoupling structures. The proposed design extends an optimized broadband SDSMA radiating element into a four-port MIMO configuration to enhance gain while maintaining wideband operation. Four orthogonally arranged SDSMA radiators integrated with a unified ground structure are employed to suppress mutual coupling without requiring defected ground structures, parasitic elements, neutralization lines, or external decoupling networks. The realized antenna occupies a compact footprint of 43 × 43 × 0.787~ mm3 on a Rogers RT/Duroid 5880 substrate ( r = 2.2). Experimental validation confirms an operating bandwidth of 26.50-33.0 GHz with a minimum reflection coefficient of -41.88 dB. In addition, port isolation remains better than -33.50 dB throughout the operating band while maintaining stable radiation characteristics, a peak gain of 18.10 dBi, and radiation efficiency exceeding 83%. The diversity performance is characterized by ECC values below 0.0055, DG approaching 9.99 dB, low TARC, stable MEG, and negligible CCL. The close correspondence between numerical and experimental observations verifies the effectiveness of the antenna configuration and demonstrates its suitability for compact, high-performance Ka-band mm-wave MIMO communication platforms.