Lithium (Li) is an essential element in modern energy production and storage devices. Technology to extract Li from seawater, which contains ~ 230 billion tons of Li, offers a solution to the widespread concern regarding quantitative and geographical limitations of future Li supplies. To obtain green Li from seawater, we propose an unassisted photoelectrochemical (PEC) Li extraction system based on an III-V-based triple-junction (3J) photoelectrode and a Li-ion selective membrane with only sunlight as an input. A light-harvesting/catalysis decoupling scheme yielded a 3J photoelectrode with excellent light-harvesting and catalysis reaction capabilities and superb stability over the 840 h of the extraction process. It allows the system to successfully enrich seawater Li by 4,350 times (i.e., from 0.18 ppm to 783.56 ppm) after three extraction stages. The overall reaction of the unassisted PEC green Li extraction system achieved 2.08 mg kJ-1 of solar-to-Li efficiency and 3.65% of solar-to-hydrogen efficiency. Photoelectrochemical (PEC) lithium extraction device is designed to explore lithium from seawater for the first time. The PEC cell with a triple-junction (InGaP/GaAs/Ge) photoelectrode and light-harvesting/catalysis decoupling scheme is constructed, offering a suitable operating potential and superb stability to the membrane-based extraction process in the seawater. The device can successfully enrich lithium by 4,350 times (from 0.18 to 783.56 ppm).