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In this study, we assessed the transformation of raw olive pomace into carbon-richmaterial using olive mill wastewater (OMWW) as the liquid medium for the hydrothermal carbonization (HTC) process. The findings were compared accordingly with the use of distilled water (DW), which is the conventional practice. The use ofOMWWas a liquidmatrix enhanced the hydrochar yield, but volatile matter, fixed carbon content, and O/C and H/C ratios followed a decreasing trend. Furthermore, for an HTC temperature of 220 °C, the use ofOMWWconsiderably increased the high heating value of the hydrochars from approximately 24.2 MJ/kg to 31.6 MJ/kg. According to the van Krevelen diagram of feedstock and derived hydrochars, dehydration was the predominant carbonization reaction for both liquid sources. Morphological characterization of both sets of hydrochars indicated the generation of specific carbon nuclei when using DW while OMWW led to the creation of hydrochar with a less homogeneous surface. Structural analysis revealed the heterogeneous aspect of the hydrochar surface with an abundance of crystallized metal-based inorganic salts. [ABSTRACT FROM AUTHOR]