As a clean and green alternative fuel to replace fossil fuels, hydrogen could be an ideal fuel for the future. Supercritical water gasification of lignocellulosic agricultural residues results in H2 production with zero CO2 emission, which makes this technique an attractive technology for hydrogen generation from biomass. Structural analyses were performed to determine the lignin, cellulose, and hemicellulose contents in feedstock. The effects of different process variables (temperature, reaction time, and feed concentration) on supercritical water gasification of Iranian rice straw (IRS) were evaluated. IRS, which has a high content of cellulose and hemicellulose, has significant potential for gaseous product generation under the supercritical water condition. The maximum H2 production of 5.56 mmol/gr of biomass was achieved at 440 °C (temperature), 20 min (reaction time), and 2 wt. % (feed concentration).