Massive quantities of waste tires are generated worldwide, posing significant challenges for disposal and recycling. In this study, waste tires were catalytically gasified in subcritical and supercritical water to investigate their degradation behavior and syngas production potential. Hydrothermal gasification represents a promising and environmentally benign pathway for converting waste tires into combustible, hydrogen-rich fuel gas. However, to the best of current knowledge, limited studies have reported the gasification of waste tires in an aqueous medium. Accordingly, this work focuses on the catalytic hydrothermal gasification of waste tires under subcritical and supercritical conditions to maximize hydrogen-rich gas production. The effects of key process parameters, including temperature, reaction time, and feed concentration, were systematically evaluated and optimized to enhance gas yields and conversion efficiencies. In addition, the effectiveness of hydroxide catalysts, such as Ba(OH)₂, Ca(OH)₂, and Mg(OH)₂, was investigated under supercritical conditions for tire degradation. Furthermore, transition-metal catalysts, including Ni/SiO₂–Al₂O₃ and Ru/Al₂O₃, were employed to promote both decomposition and reforming reactions, and their effects on gas yields were examined. The process conditions were varied over a temperature range of 325–625 °C, reaction times of 15–60 min, and feed concentrations of 5–20 wt%. Optimal performance was achieved at 625 °C, 60 min, and 5 wt% feed concentration, yielding the highest total gas production (34 mmol/g), hydrogen yield (14.4 mmol/g), and carbon gasification efficiency (42.6%). The application of both homogeneous and heterogeneous catalysts significantly enhanced hydrogen production, following the order: Ni/SiO₂-Al₂O₃ (19.7 mmol/g) > Ru/Al₂O₃ (17.9 mmol/g) > Ba(OH)₂ (16.9 mmol/g) > Ca(OH)₂ (16.7 mmol/g) > Mg(OH)₂ (15.4 mmol/g). These findings demonstrate that waste tires are a viable resource for producing hydrogen-rich fuel gas via supercritical water gasification.