In this study, conversion of seaweeds into hydrochars was investigated with the aim of obtaining a renewable energy feedstock. The seaweeds Fucus serratus and Alaria esculenta, and a mixture of seaweeds, mainly consisting of Cystoseria sp. and Laurencia sp., were subjected to hydrothermal carbonization (HTC) in subcritical water at three different temperatures: 200, 225, and 250 degrees C. Fuel characteristics and chemical properties of the derived hydrochars were determined using the standard fuel analysis and spectroscopic methods. The combustion behavior of seaweeds and hydrochars was examined via nonisothermal thermogravimetric analysis under air atmosphere. The seaweed-derived hydrochar yields were lower than those of the lignocellulosic-derived hydrochar yields in the literature. Hydrochars derived from Fucus serratus and Alaria esculenta became increasingly similar to lignite with higher process temperature. Fucus serratus-derived hydrochars had the highest calorific value due to their higher carbon content and significantly lower ash content. HTC converted the seaweeds to hydrochars with improved combustion characteristics observed by lower burnout temperature and higher reactivity during combustion. The slagging index values of hydrochars implied medium or high slagging potential during combustion. On the other hand, HTC resulted in hydrochars with reduced fouling index implying medium fouling potential during their combustion due to the significant removal of alkali metals.