Abstract Hydrochar is the product of the hydrothermal carbonization process, which is able to convert biomass into a carbon-rich material at mild hydrothermal conditions. This article is aimed at exploring a novel possibility, that is converting hydrochar into a valuable fuel gas by means of another hydrothermal technology: supercritical water gasification (SCWG), which operates at higher temperatures and pressures. Tests were performed in 5 ml micro-autoclaves with a hydrochar concentration of 15 wt.%. Residence times between 1 h and 16 h were adopted. The results showed that hydrochar derived from maize silage can still be gasified, producing a gas rich in CO2 and CH4. The data here obtained were also compared with literature data concerning the SCWG of glucose and beech sawdust, highlighting the main differences in terms of solid, liquid and gaseous yields. The solid material obtained after SCWG was observed by means of scanning electron microscopy after different residence times, highlighting the formation of coke spheres. Finally, the effect of the addition of an alkali catalyst (K2CO3) was tested and discussed.