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Biomass-derived energy is gaining more attention due to environmental issues and increasing energy demand. To ensure the sustainability of fossil energy substitution using biomass, diversification of sources, including marine organisms, is vital. Among various types of marine biomass discussed in the literature, the utilization of green algae Ulva lactuca for energy generation is still rare globally. Therefore, this study aims to investigate the potential of green fuel (syngas and hydrochar) production from U. lactuca (Chlorophyta) via sub-critical water gasification (SbWG). The experiments were conducted using a batch reactor at varying temperatures (300, 350, and 400 °C), reaction times (30, 60, and 90 min), and feedstock concentrations (1 and 5 wt%). The effect of temperature on gas composition was examined in detail. The results revealed that increasing temperature from 300 to 400 °C leads to an increase in the H 2 content significantly from 2.21 % to 8.09 % within 90 min. However, increasing feedstock concentration from 1 to 5 wt% reduces the H 2 fraction due to suppression of the steam reforming and water-gas shift reactions. Based on the ultimate analysis, the high severity of operating conditions leads to lower O/C and H/C atomic ratios owing to dehydration and decarboxylation reactions. It was confirmed by scanning electron microscope (SEM) analysis that more void structures existed in hydrochar than the algal feedstock. The SbWG process at varying temperatures and times can increase the energy contents of U. lactuca by over 47 %. Intriguingly, hydrochar obtained at 400 °C exhibited higher HHVs (i.e., 21.75–22.93 MJ kg−1) than typical low-ranked coals, making hydrochar more potential to be used as solid fuels. Finally, a reaction model was deduced, and the decomposition of U. lactuca was confirmed to follow the Arrhenius behavior. Simultaneous production of green fuels (syngas and hydrochar) from green algae (Ulva lactuca) via sub-critical water gasification. [Display omitted] • Simultaneous production of syngas and hydrochar from U. lactuca • Increasing temperature leads to an increase in the H 2 content. • Hydrochar exhibited an energy potential compared to the low ranked coals. • HHV of hydrochar was obtained in the range of 21.75–22.93 MJ kg−1. • The plausible reaction pathways and kinetics were deduced in detail. [ABSTRACT FROM AUTHOR]