All-solid-state batteries (ASSBs) are one of the most promising candidates for next-generation energy storage. In particular, ASSBs with sulfide solid-state electrolytes (SEs) offer high conductivity, rivaling those of liquid electrolytes (LEs). However, a fundamental understanding of thermal failure of sulfide-based ASSBs is lacking. Herein, we unprecedently revealed two distinct thermal runaway (TR) mechanisms of sulfide-based ASSBs, namely the gas–solid and the solid–solid reactions. Contrary to the prevailing wisdom, both glassy-ceramic (Li3PS4 and Li7P3S11) and crystalline (Li6PS5Cl and Li10GeP12S2) SEs exhibited significantly larger heat generation than LEs with the delithiated LiNi0.8Co0.1Mn0.1O2 (NCM) cathode revealed by DSC-MS characterization, and confirmed in composite cathode pellets. Driven by the gas–solid reactions, the glassy-ceramic SEs were oxidized by the O2 released from the NCM cathode at approximately 200°C, resulting in tremendous heat and toxic SO2 gas generation. On the contrary, the crystalline sulfide SEs remained stable against lattice O2 at 200°C without SO2 generation and showed solid–solid reactions with the decomposition products of the NCM cathode (transition-metal oxides, etc.) at 300°C. Ex-situ characterizations revealed the generation of different decomposition products at elevated temperatures, indicating the different failure routes of sulfide SEs with NCM. The proposed mechanisms delineate the relationship among the sulfide SE structures, gas-driven crosstalk reactions, and interfacial reactions due to solid–solid contact during the TR of sulfide-based ASSBs. Our study highlights the need for fundamental studies on the safety of ASSBs and sheds new light on the design principles of emerging materials for intrinsically safe batteries.