The proposed work presents a comprehensive numerical study of a geometrically tunable terahertz (THz) metamaterial absorber for refractive index sensing applications. The absorber’s response is evaluated under simultaneous variation of both the surrounding dielectric environment, represented by a refractive index ranging from n = 1.2–1.8, and its structural geometry. Its influence on the absorber’s resonant behaviour is systematically analysed. Resonance characteristics, including resonant frequency, quality factor (Q), sensitivity, and figure of merit (FOM), are extracted for both lower- and higher-order modes exhibiting absorption greater than 0.9. Results demonstrate that increasing the refractive index leads to a pronounced red-shift in the resonant frequency, accompanied by improvements in Q and FOM. For lower resonant modes, a maximum sensitivity of 21.99 THz/RIU and FOM of 46.04 are achieved, while higher-order modes yield a sensitivity of 3.09 THz/RIU and FOM of 5.34, underscoring the potential of the proposed absorber for high-resolution, reconfigurable THz sensing applications