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Summary: Hydrothermal pretreatment is an effective method to increase the energy density of biowastes with high moisture content. Since the hydrothermal pretreatment slightly modifies the lignocellulosic structure, the thermochemical conversion behavior of the biomass also changes. In the present study, grape pomace (GP) and fermentation sludge (FS) were hydrothermally pretreated at 220°C and 90 minutes. The fuel properties, pyrolysis, and combustion behaviors of the corresponding hydrochars were compared. After pretreatment, the fixed carbon content (wt%) of GP increased by 7.2% and that of FS by 16.8%, while the ash content (wt%) of GP decreased by 77.6% and that of FS by 38.8%. Due to a decrease in both H/C and O/C, the heating value of GP and FS increased by 11% and 36%, respectively. Upon pretreatment, ignition temperatures were shifted from 245.8°C to 295.8°C for GP and 273°C to 279°C for FS. Burnout temperatures were also shifted to 725°C for GP, and 700°C for FS. The increase in activation energies after pretreatment (from 20.4 to 32 kJ/mol for GP, from 24.3 to 36.3 kJ/mol for FS) showed that the pyrolysis process was slowed down. The main volatile compounds (compounds containing C═O and CO2) detected from Fourier transform infrared (FTIR) in the pyrolysis of GP were formed at higher temperatures after hydrothermal pretreatment. In the pyrolysis of FS, the main volatile compounds (CO2 and CH4) were seen at lower temperatures after pretreatment. According to the fast pyrolysis results in Py‐GC/MS, hydrothermal pretreatment caused an increase in esters and a decrease in fatty acids and ketones. Phenols were the dominant group in the pyrolysis products of GP after pretreatment, while other hydrocarbons, esters, and aromatic hydrocarbons were seen in FS besides phenols. Novelty Statement: Hydrothermal carbonization, which is preferred for upgrading the fuel properties, is applied prior to combustion mostly. In our study, we preferred it as a pretreatment before pyrolysis to modify the pyrolysis products of two different wastes: grape pomace and fermentation residue. Additionally, biofuel production from the fermentation residue from the production of alcoholic beverages has been studied for the first time. [ABSTRACT FROM AUTHOR]