As the field of terahertz (THz) photonics advances, we present a monolithic gallium arsenide (GaAs) disk-shaped whispering gallery mode resonator that has potential as a component in THz nonlinear optics. GaAs is a material with significant optical nonlinearity which can be enhanced when the crystal is shaped into a microdisk resonator. A 4-mm-disk-resonator was fabricated using single-point diamond turning and was characterized to obtain a quality (Q) factor of 2.21k at ∼150 GHz and 1.41k at ∼300 GHz. We also demonstrated the blue-shifting of up to ∼0.3 GHz of the THz modes using a block of metal. This post-fabrication degree of freedom could be useful for phase-matching requirements for nonlinear optical processes, such as detection based on optical up-conversion of THz radiation. This proof-of-concept demonstration can pave the way for the implementation of a compact, tunable and efficient device which could be integrated into nonlinear photonic platforms for THz generation, manipulation and detection.