For next-generation 6G wireless communication applications, a small, rectangular Multiple Input Multiple Output (MIMO) antenna that operates in the terahertz (THz) frequency range is proposed in this paper. Because of its exceptionally high-speed transmission capabilities, graphene is used in the design of the antenna. To improve the isolation between the two radiating elements, decoupling techniques using Defected Ground Structure (DGS) were used. A detailed parametric study of the DGS geometry was carried out to evaluate its effect on mutual coupling and to optimize antenna isolation performance. To examine the importance of employing these techniques, a parametric study is conducted. The designed MIMO antenna’s performance is assessed in terms of various metrics using the CST Microwave Studio simulator. The achieved resonant frequency is 4 THz, antenna efficiency 95%, with a smaller antenna physical size compared to other antenna designs in the literature. These improvements highlight the potential for antennas to be integrated into dense 6G platforms that require an efficient and compact form factor.