Real-time chemical sensing is critical for industrial quality control and safety. However, most terahertz (THz) photonic crystal fiber (PCF) sensors remain limited to material characterization rather than practical transductors. Here, we introduce Sagnac interferometry to the terahertz domain as a transduction platform for sensing, demonstrating a numerical proof-of-concept liquid sensor using a highly birefringent porous-core photonic crystal fiber infiltrated with an ethanol–water mixture analyte as a proof-of-concept. By translating birefringence modulation into spectral shifts, our simulations show that the sensor achieves sensitivities up to 549 MHz per % v/v, with excellent polynomial correlation ( ) across a wide concentration range, when operating in the 0.56–0.70 THz band. This work establishes a versatile and straightforward interferometric platform in the THz gap, highlighting the potential of this interferometric approach for future chemical and biological applications in the terahertz range.