All-solid-state sodium batteries utilize earth-abundant elements and are sustainable systems for large-scale energy storage and electric transportation. Replacing flammable carbonate-based electrolytes with solid-state ionic conductors promotes battery safety. Using solid-state electrolytes (SEs) also eliminates the need for packing when fabricating tandem cells, potentially enabling further enhanced energy density. Na₃SbS₄, a Na⁺ conductor, remains stable in dry air and shows high Na⁺ conductivity (σ ≈ 1.0 × 10^–3 S/cm) and is thus a promising SE for applications in sodium batteries. However, upon repeated electrochemical cycling, Na₃SbS₄-containing Na batteries exhibit decaying capacity and limited cycle life, which is likely associated with the decomposition of Na₃SbS₄ at the electrode/electrolyte interface. This work presents an in-depth analysis of the decomposition chemistry occurring at the Na₃SbS₄/anode interface using combined in situ Raman and post-mortem characterization. The results indicate that the SbS₄^3– counterion is electrochemically reduced when experiencing Na⁺ reduction potentials, and this reduction chemistry likely follows multiple pathways. The observed reduction products include SbS₃^3–, the Sb₂S₇^4–dimer, the NaSb binary phase, and Na₂S. We also observed the irreversibility of the decomposition and, as a consequence, the accumulation of the degradation products over cycles. Also notable is the heterogeneity of this degradation chemistry across the interface. Through the spectroelectrochemical characterizations, we reveal the possible mechanisms of the Na₃SbS₄ decomposition at the solid electrolyte/anode interface in an operating device.