A dual-band decoupling strategy via artificial transmission line (TL) for closely spaced two-element multiple-input multiple-output (MIMO) antenna is proposed, which consists of two composite right-/left-handed TLs for dual-band phase shifting and a cross-shaped TL for susceptance elimination to counteract the real and imaginary part of the mutual coupling coefficient S21 at dual frequency bands, respectively. The decoupling principle and detailed design process of the dual-band decoupling scheme are presented. To validate the dual-band decoupling technique, a closely spaced dual-band MIMO antenna for 5G (sub-6G frequency band) utilization is designed, fabricated, and tested. The experimental results agree well with the simulation ones. A dual-band of 3.40 GHz-3.59 GHz and 4.79 GHz-4.99 GHz (S11&S22 < -10 dB, S12&S21 < -20 dB) has been achieved, and the mutual coupling coefficient S21 is significantly reduced 21 dB and 16.1 dB at 3.5 GHz and 4.9 GHz, respectively. In addition, the proposed dual-band decoupling scheme is antenna independent, and it is very suitable for other tightly coupled dual-band MIMO antennas.