A compact millimeter-wave (mmW) multiple-input multiple-output (MIMO) antenna using Array-antenna Decoupling Surface (ADS) for 5G applications is reported. Initially, a single element antenna is designed for 31 GHz using a rectangular patch with a footprint of 5 × 4 × 0.508 mm3. By arranging the unit element in a linear fashion along the x-direction, the two-element MIMO structure is realized. The uniqueness of this design is the integration of the Sierpinski fractal slot and non-resonating rectangular slots onto the radiating patch, which enhances the impedance matching. To minimize the mutual coupling, an ADS is introduced with an isolation level exceeding 40 dB at operational frequency of 31 GHz has been achieved. The proposed 1 × 2 MIMO antenna attains a bandwidth of 1.06 GHz from 30.45–31.51 GHz and simulated gain of 8.32 dB. The overall performance of the MIMO system is evaluated through its diversity parameters. The results show envelope correlation coefficient < 0.0033 and mean effective gain in the range of 0.49 dB have been realized. Finally, the suggested MIMO antenna demonstrates strong viability for fifth-generation communication systems at mmW band due to its compactness, high gain, and diverse performance.