Abstract Metasurfaces that support bound states in the continuum (BICs) enable extreme spectral selectivity by suppressing radiative leakage; controlled symmetry breaking converts ideal BICs into quasi-BICs with finite yet ultra-high quality factors. Here, we design and numerically validate an all-dielectric silicon metasurface on SiO₂ that, under normal incidence, supports five quasi-BIC resonances in the 0.8–1.0 THz band. The metasurface unit cell comprises two semi-elliptical silicon elements; introducing a narrow slit in one element perturbs the symmetry and couples the otherwise dark modes to free space. Transmission and eigenmode analyses reveal five sharp resonance dips in the 0.8–1.0 THz band. We harness the quasi-BIC resonances excited under TE polarization for refractometric sensing by placing an analyte layer above the metasurface and quantify performance vs. analyte refractive index (1.4–1.5) and thickness (0–150 μm). The device attains a maximum sensitivity of 169.6 GHz RIU⁻¹ and a figure of merit (FOM) of 3.818 × 10 3 RIU −1 and a Q-factor of 3.42 × 10 4 owing to strong near-field confinement in the slit-perturbed element. The straightforward geometry, multi-resonant operation, and compatibility with dielectric microfabrication provide a practical route to multi-channel THz sensors. Our results establish design guidelines for engineering quasi-BICs at THz frequencies and for tailoring their spectral response to maximize sensing performance.