Terahertz (THz) fields offer a powerful approach for controlling nonlinear optical processes and manipulating surface/interface responses on ultrafast timescales. Here, we demonstrate that THz-driven nonlinear polarizations generated at spatially separated interfaces can be harnessed to create controllable interference in a finite-thickness polar crystal. Using single-cycle THz pulses to pump a ZnO crystal and an infrared beam to probe the response, we observe pronounced second-harmonic generation (SHG) interference fringes that originate from the coherent superposition of signals from the front and rear surfaces. Without THz excitation, the spectral interference fringes arise from the phase accumulation due to refractive-index dispersion between fundamental and second-harmonic waves propagating through ZnO. Upon THz excitation, THz-field-induced second-harmonic generation (TFISH) actively reshapes the interference pattern, introduces additional delay-dependent modulation, and tilts the interference fringes in the energy-delay map, demonstrating direct control over the interference via the THz field. Crucially, we show that the THz field governs the fringe contrast and temporal gating, while the optical dispersion fixes the energy-domain fringe spacing, enabling a clear separation of their respective roles. This THz-driven surface-to-surface SHG interference serves as a sensitive probe of interface nonlinearity and dispersion, providing a compact interferometric platform for refractive-index sensing, and ultrafast optical modulation in polar materials—opening new avenues for THz–optical hybrid devices with performance metrics rivaling those of conventional approaches.