A wideband 8-antenna Multiple-Input Multiple-Output (MIMO) array covering 3.3-7.1 GHz for Fifth-Generation (5G) sub-7 GHz and New Radio Unlicensed (NR-U) applications in metal-rimmed smartphones is presented in this paper. In this design, the open slot loaded metal rim is directly fed by microstrip line. Hybrid Inverted-F Antenna (IFA) and slot modes are generated. Utilizing impedance matching and reactance loading, the two modes are moved and combined, so as to achieve size reduction and wideband coverage. The size of the ground slot (clearance) is only 12.4 mm × 1.5 mm (0.136 λ×0.016 λ at 3.3 GHz). The proposed MIMO antenna array is fabricated and measured. Results show that in the desired wide frequency band, the proposed design can achieve desirable antenna performances, including isolation >11 dB, total efficiency >47%, and calculated Envelope Correlation Coefficient (ECC) <0.09. Besides, antenna gain, radiation pattern and calculated ergodic channel capacity are demonstrated as well. The proposed metal-rim-integrated MIMO antenna array features small size, simple structure and wide bandwidth. It can be a good application-oriented design in next-generation 5G mobile communication.