Abstract Crude bio-oil production through hydrothermal liquefaction (HTL) of biomass requires a tremendous amount of water. Current operations commonly use freshwater as the processing medium that is already scarce across the globe. Replacing freshwater by seawater holds promise, however has not been explored yet. In this study, four types of biomass with largely different biochemical compositions were chosen as representative feedstocks, including microalgae (Chlorella sp.), marine macroalgae (red seaweed), food processing waste (spent coffee grounds) and lignocellulosic waste (sawdust). Microwave-assisted HTL of each feedstock was conducted under varying NaCl concentrations, 0 wt%, 1.5 wt% and 3 wt%, representing freshwater, brackish water and seawater respectively. Change in NaCl concentration was not influential for HTL of Chlorella sp., red seaweed and spent coffee grounds in terms of biocrude yield and quality. However, in the HTL of sawdust (containing more fibers than the other three feedstocks), a much lower biocrude yield (7.9 wt%, daf) was obtained in seawater than in freshwater (27.1 wt% daf), presumably due to the condensation of intermediates generated from decomposed carbohydrate and lignin in the presence of NaCl. This assumption was further verified by a computational simulation based upon Density Functional Theory, indicating that the condensation of phenolics-aldehydes was kinetically favored in the presence of NaCl.