This paper presents, a Graphene-based octagonal three-element series feed (OTESF) array antenna designed for terahertz (THz) communication at short distances. The OTESF array antenna comprises four layers, utilizing gold for both ground and patch. The substrate consists of quartz (SiO4) with a thickness of 140 μm and a 10 nm thick graphene layer is incorporated after the patch layer in this array antenna design. The unit cell of the OTESF array antenna exhibits excellent performance, showcasing a reflection coefficient below-10 dB within the 0.285 to 0.535 THz frequency range. This achievement corresponds to a significant bandwidth of 250 GHz. additionally, the peak realized gain is measured at 5.25 dBi, making the OTESF array antenna a highly promising candidate for applications in Multiple Input Multiple Output (MIMO) setups. In dual Port (DP) mode configurations (1×2 MIMO), where the OTESF array antenna elements are organized orthogonally. The DP MIMO antenna features a cross-sectional area with dimensions of 1225.71 μm × 2155.71 μm. The designed MIMO array attains a bandwidth of 0.256 THz, operating in the frequency ranges from 0.286 to 0.542 THz and realized gain of 7.5 dBi, and the isolation loss is consistently kept below-20 dB.