A four-element multiple-input multiple-output (MIMO) antenna based on a modified Complementary Split Ring Resonator (MCSRR) is presented in this paper for dual-band 5G smartphone applications. An inverted L-shaped radiator is used with MCSRR as an open stub in the ground, where MCSRR is responsible for dual operating bands and enhances the impedance matching. The MCSRR as an open stub in the ground plane creates a notch band that minimizes the interference in 5G wireless communication. The four elements of the antenna are placed in such a way that minimum isolation between antenna elements is obtained, 16.5 dB, without any decoupling, whereas more than 20 dB isolation is achieved by using T-shaped decupling. The antenna achieves dual 10 dB bandwidths from 3.40 GHz to 3.625 GHz and from 3.90 GHz to 4.525 GHz. Envelop correlation coefficient (ECC) is extracted from far-field results to analyse the MIMO antenna performance in practical design consideration.