In this paper, a wideband eight-element antenna array operating in the 5G (Fifth-generation) NR (New Radio) frequency bands N77/N78/N79 (3300-5000 MHz), LTE (Long Term Evolution) band 46 (5150-5925 MHz), and EU (European Union) 5G band of 5900 to 6400 MHz for 8 × 8 MIMO (Multi-Input Multi-Output) operation in the future 5G mobile terminal is presented. The proposed eight antenna elements are all co-coupled-fed IFA (Inverted-F antenna)/loop structure, and are divided into two same sub-arrays which printed on the inner surface of two long frames of the mobile terminal, respectively. All the simulated and measured results present consecutive 6-dB impedance bandwidth of 3200 to 6500 MHz, acceptable isolations (better than 12 dB), and good total efficiencies (more than 50%). Besides, the proposed wideband 8-antenna MIMO system has also shown good MIMO performances with ECCs (envelope correlation coefficient) are all lower than 0.08, and ergodic channel capacities are all higher than 37.8 bps/Hz within the entire desired operation bands. In addition, the hand phantom effects are also investigated. The prototype of the proposed wideband eight-element antenna array is fabricated and measured, and a quite good agreement between simulation and measurement is obtained.