This paper proposes catalytic hydrothermal reforming (CHTR) for producing substitute natural gas (SNG) directly from a lignin in aqueous alkaline media, which can fully dissolve the lignin but stabilize it, lowering the reactivity toward water. Among catalysts preliminarily tested, activated-carbon-supported ruthenium (Ru/AC) catalysts showed the highest activity in terms of reduction of the total organic carbon concentration (TOC) in the aqueous solution. CHTR of a lignin was performed employing a 5000 ppm TOC solution of 0.1 M Na2CO3 in a continuous reactor. The presence of Na2CO3 in the solution enabled delivery of the lignin, which is poorly soluble in water, to the reactor as well as suppression of char formation during CHTR A Ru/AC showed its ability to maintain 98.6% conversion of the lignin at 350 degrees C even under the alkaline environment for a duration of at least 10 h, with colorless effluent liquid containing 70 ppm TOC of organic carbon. A low content of Ru in the Ru/AC resulted in an insufficient yield of gas, because of the deposition of a portion of the lignin as coke over the catalyst, while 20 wt % Ru was enough for the full conversion into gas composed mainly of CH4, With cold gas efficiency (CGE) of 100.4% on a higher heating value (HHV) basis. The resulting aqueous solution of Na2CO3 was ready to be reused for CHTR after removal of carbonate ions derived from the lignin by aeration.