A wideband multiple-input multiple-output (MIMO) antenna with high isolation for fifth-generation (5G) new-radio (NR) smartphone employment operating at a range from 3.3–5 GHz is presented in the paper. The wideband antenna is comprised of eight identical radiation units and applies defected ground structure (DGS) to offer higher isolation between antenna units. Each of the antenna elements is formed of a C-shaped microstrip line, two triangle-shaped strips and a rectangular strip. The measurement results present that the transmission coefficients of the presented antenna are smaller than −18 dB in the required wideband, and the envelope correlation coefficient (ECC) values are better than 0.005. The realized gain and efficiency of the designed antenna are about 3.5 dBi and higher than 75%, respectively. The volume of this total antenna is 150 × 75 × 7 mm³, and the measurement of each antenna unit is merely 6.8 mm × 7 mm × 0.8 mm ($0.077 \lambda \times 0.08 \lambda \times 0.009 \lambda$, $\lambda$ is the wavelength of 3.4 GHz). The presented MIMO antenna is suitable for 5G new-radio (NR) smartphone applications.