Hydrogen gas has been successfully produced from cultivated microalgae biomass by supercritical water gasification (SCWG). This paper describes SCWG of natural cyanobacterial biomass for hydrogen production at low temperatures (<500 degrees C). Natural decomposition of cyanobacteria biomass was first studied to achieve prospective H-2 production. Transformation of carbon, nitrogen, and phosphorus and the degradation of microcystins were also clarified for the nature and safety assessment of SCWG products. Cyanobacteria would remain an ideal SCWG feedstock for H-2 production such as cultivated microalgae biomass, and it could be gasified as 2.92 mol/kg of H-2 yield without any catalyst addition at 500 degrees C. Liquid products with above 2800 mg/L ammonia-nitrogen and 100% removed microcystins had great potential for producing high-quality liquid fertilizers. C (50.50%) was concentrated into a solid residue as char and tar. The percentage of P combined with Ca2+ and turned into authigenic apatite was as high as 72.25%. The synergistic effects of ash, N element, or proteins, and fixed carbon content make cyanobacteria a more optimistic feedstock for H2 production than some other types of organic wastes. The SCWG technology provides an effective method of gasification of waste cyanobacteria for high-value energy utilization.