Waste seaweed that is collected at coastal regions of maritime provinces in Canada is creating ecological problems as it promotes an anoxic event, which produces nearly zero dissolved oxygen in water along with hydrogen sulfide emission. The work done in this study attempts to address this issue by producing a coal-like solid hydrochar and nutritious liquid slurry (processed water) by employing a rather recent thermo-chemical process called hydrothermal carbonization (HTC) on the seaweed. The HTC was carried out in a batch reactor system for three different reaction temperatures, 180 °C, 200 °C, 220 °C, and three different reaction times, 30, 60, and 120 min. Each of the produced hydrochars was characterized by different analytical methods. The effects of the process conditions on the yield and the properties of the hydrochar and process water were examined. The hydrochar produced at 220 °C and 120 min showed the highest carbon content (48.5%) and heating value (18.93 MJ/kg). The energy density and carbon to nitrogen (C/N) ratio in the hydrochar increased significantly as compared to raw seaweed. Moreover, HTC reduced the ash yield and volatile compounds of the seaweed. Thus, hydrochar can be used as a fuel for direct combustion, in soil remediation, or in carbon sequestration applications.