This work explores the potential of hydrothermal carbonization (HTC) for converting wet biowaste into energy-rich solid fuels. Various organic residues—including spent coffee grounds, olive pomace, grape marc, hemp, and digestate—were treated at 220 °C for 1 hour. The aim was to assess the impact of HTC on fuel properties through elemental and proximate analysis, and higher heating value (HHV). Results showed a consistent increase in carbon content and a reduction in H/C and O/C ratios, indicating effective dehydrogenation and decarboxylation. These changes led to higher fixed carbon content and improved HHVs, up to 30.41 MJ/kg for olive pomace-derived hydrochar. The Van Krevelen diagram confirmed the transformation of hydrochars toward coal-like materials, especially lignite and peat regions. Spent coffee grounds and olive pomace produced hydrochars with the best energy profiles and solid yields above 50%, while digestate showed limited carbonization and low HHV. Overall, HTC demonstrated high effectiveness in upgrading biowaste into solid fuels, supporting its role in biowaste valorization within circular bioeconomy frameworks.