We propose a linear to right-handed circularly polarized tunable terahertz antenna. Polarization-switchable graphene is used to enable tunable polarization conversion and circular polarization. Polyimide is used as a substrate material for designing THz components due to its low absorption. By adjusting the graphene chemical potential to be between 0 eV to 0.8 eV, the polarization state in the 0.55 THz band can be obtained without changing the physical geometry. The proposed antenna has a significant potential for use in tunable terahertz devices and related applications.