In the network era of radio waves, as the need constantly continues to grow at the current level, new radio frequency bands must be found. With 5G technology being replaced, the terahertz (THz) frequency band is expected to be the most suitable resource to power wireless networks within the range of 0.1 THz up to 10 THz. This article describes a model of an ultrawideband 2 × 2 multiple-in, multiple-out (MIMO) antenna for THz applications. As the frequency moves upward, the cross-coupling of the lead elements between elements becomes an inevitable problem that brings undesirable effects on the overall efficiency of the antennas. Hence some practices have been introduced to reduce the mutual coupling impact on MIMO antenna measurements like high isolation, parallel configuration, parasitic structures, and feeding slots. The model presented in this paper comes with a radiation radiator of a 133 × 255 μm<sup>2</sup> rectangular shape with a circular cut in the middle. The designed structure has a large bandwidth typically ranging from 5.5 THz to 10 THz, and hence, it can be utilized in high-speed wireless communication networks and in working with high bit rates. The antenna performance is also conducted in terms of a number of factors like channel capacity loss and total active reflection coefficient. Following this, it is compared with other existing designs. By comparing the resulting model and existing standards in different parameters, it proves that the model is superior to existing designs.