This research paper presents, an octagonal shaped graphene based terahertz patch antenna is designed for indoor communications. For a unit cell, a single graphene layer with a thickness of 10 nm has been layered over a quartz (SiO4) substrate with a thickness of 100 µm to increase the effectiveness of the multiple-input and multiple-output (MIMO) antenna. The MIMO antenna is chosen to operate at a frequency of 0.3 THz. For a quad-port MIMO configuration, the element cells have been arranged orthogonally. The total cross-sectional area of the terahertz (THz) MIMO antenna is 1325 × 1325 µm2. The suggested MIMO antenna device achieves a bandwidth of 113 GHz over a frequency range of 0.257–0.370 THz. Moreover, the mutual coupling coefficient is less than − 15 dB. The designed THz MIMO antenna’s diversity performance is analyzed and the obtained diversity matrices are the diversity gain (DG) (< 9.96 dB), envelope correlation coefficient (ECC) (< 0.01), channel capacity loss (CCL) (< 0.35 bps/Hz), and total active reflection coefficient (TARC) (< − 10 dB), respectively.