Abstract <p>In the valorisation of wet biomass, the improvement of dewatering capabilities plays an important role, due to copious amounts of heat required for thermal drying. This work aimed at determining of influence of hydrothermal treatment on mechanical dewatering of wet brewer’s spent grain (BSG) and assessing its hydrophobic character, by performing mechanical dewatering in a hydraulic press, as well as molarity of ethanol droplet tests of dry samples. Hydrothermal processing at 170 °C resulted in the lowest moisture content of the valorised material after mechanical dewatering, whereas an increase in the process temperature to 200 °C resulted in higher final moisture content, attributed to secondary chars increasing the hydraulic resistance of the filtration cake. HTC treatment changed hydrophylic biomass into moderately hydrophobic fines and slightly hydrophobic coarse particles, showing the influence of particle size on the contact angle. HTC treatment of BSG resulted in an increase in carbon content and improvement in terms of grindability of the material.</p> Graphical Abstract <p></p> Highlights <p><UnorderedList Mark="Bullet"> <ItemContent> <p>HTC treatment changed hydrophylic BSG into hydrophobic hydrochars.</p> </ItemContent> <ItemContent> <p>Fines were moderately hydrophobic and coarse particles were slightly hydrophobic</p> </ItemContent> <ItemContent> <p>Hydrothermal processing at 170 °C resulted in the lowest moisture content</p> </ItemContent> <ItemContent> <p>Secondary chars increased hydraulic resistance of the filtration cake</p> </ItemContent> </UnorderedList></p>