CO2 captured from various sources can be converted into renewable natural gas (RNG) via the Sabatier reaction (thermocatalytic hydrogenation of CO2). The highly exothermic nature of the Sabatier reaction brings certain challenges to the design and operation of Sabatier reactors, with thermal management being one of the main issues. In this study, we report the experimental, lab-scale proof-of-concept of an autothermal Sabatier reactor. We demonstrate a completely autothermal operation of the air-cooled, stainless-steel reactor using a commercial Ni catalyst. The effects of pressure, feed temperature, space velocity and cooling rate were investigated using three prototypes with different sizes and configurations. The maximum CO2 conversion of 93.5% with 100% generation selectivity to CH4 was achieved in a 10′′-length reactor, over 120 h of a continuous, stable operation (at 2400 L kg−1 h−1, without any reactor heating or feed preheating).