Hydrochars derived from macroalgae Sargassum horneri were characterized physically and chemically to elucidate their potential as a valuable resource. Hydrochars were prepared by hydrothermal carbonization (HTC) of Sargassum horneri at temperatures of 180–210 °C with citric acid. The hydrochars were found to form mainly through a dehydration reaction pathway and had carbon contents of 36.8–50.5% and higher heating values of 19.0–25.1 MJ kg–1. The BET surface area of hydrochars remained low, in the range of 0.6–31.8 m2 g–1. On the basis of Taguchi’s experimental design, reaction temperature, reaction time, and particle size of feedstock were found to be the most important control factors for the chemical and physical properties of the hydrochars.