Hydrothermally carbonized organic materials (furfural, glucose, starch, cellulose and eucalyptus sawdust) have been used as precursors to produce high-surface area carbons. The synthesis methodology comprises two steps: (i) hydrothermal carbonization of organic materials and (ii) chemical activation with KOH as activating agent. In this way, activated carbon materials with a high surface area (up to 2700 m² g⁻¹) and narrow micropore size distribution in the supermicropore range (0.7–2 nm) are produced. The textural properties of the activated carbon products can be easily tuned by modifying the activating conditions (i.e., the activation temperature and the amount of KOH used). The activated carbon materials exhibit high hydrogen uptakes, up to 6.4 wt%, and large isosteric heats of adsorption, up to 8.5 kJ mol⁻¹. In particular, the hydrogen storage density of the carbons is high and ranges between 12 and 16.4 µmol H₂ m⁻². The hydrogen storage density is closely related to the pore size of the carbons, with small micropores (ca. 1 nm) favouring a high density. Taking into account the high hydrogen storage capacities of these materials, as well as the simplicity of their synthesis procedure and the ready availability and low-cost of the raw precursors, it can be concluded that these activated carbons constitute a promising adsorbent for hydrogen storage.