Microalgae are a resourceful candidate for the synthesis of value-added products such as biofuels, biopolymers, etc. Microalgae paves way for a circular economy as it has the eligibility to grow in wastewater and produce biomass. Microalgae-based biofuel production is costlier when compared to conventional fuels as it needs techniques for biomass culturing and processing. This chapter covers the hydrothermal gasification (HTG) of algae biomass for bio-hydrogen production. Microalgae are third-generation feedstocks that help in carbon dioxide sequestration, high biomass productivity, cultivable in wastewater, etc. Sustainable energy development can be achieved by reducing carbon dioxide emissions during the conversion of algae biomass into bioenergy. This chapter covers the two major mechanisms (supercritical gasification and steam reforming) involved in bio-hydrogen production via the HTG process. Further research is needed in the safe storage, transportation, and distribution of bio-hydrogen as an alternate source of energy. © 2023 Elsevier B.V., All rights reserved.