Photocatalytic water splitting is a green technology for sustainable hydrogen evolution. To improve photon-to-electron conversion efficiency, the design and development of efficient, stable, and full-spectrum responsive photocatalysts has attracted increasing attention. Many different classes of materials can be used to harness solar photons for photocatalysis, each having their advantages and drawbacks. Compared to inorganic semiconductors, organic semiconductors are rich in π electrons and can be readily modified, allowing for facile control of the optic (absorption region and intensity) and electronic (energy structure) properties, as well as mechanistic pathways. However, photogenerated charge carriers cannot be effectively employed owing to subpar charge carrier transport properties, which arise from the low concentration and low mobility of free charge carriers in organic semiconductors. Appropriate changes in the molecular structure of the organic semiconductors can allow for sunlight utilization across the full visible region and even the infrared region. By controlling the nature of stacking, organic photocatalysts with different compositions, dimension (0, 1, 2, 3), size, and crystallographic orientation can be harnessed to increase sunlight utilization and charge separation efficiencies. By optimizing these properties, the overall photoelectric conversion efficiency and hydrogen production efficiency can be improved. However, the mechanisms of redox reactions in organic semiconductor photocatalytic systems remain unclear owing to the complex nature of the processes and difficulties in study design. Herein, the physical and chemical processes of organic semiconductors are discussed from the perspective of light harvesting, photoexcited charge separation, and surface reactions. The preparation methods of organic semiconductor nanostructures are summarized and the progressive development of organic nanostructures for photocatalytic hydrogen evolution is systematically reviewed. Typical organic semiconductor materials, including perylene diimide, porphyrin, phthalocyanine, fullerenes, graphitic carbon nitride (g-C3N4), and other conjugated polymers, are highlighted. Moreover, modification strategies for optimizing optical and electrical properties at the molecular or aggregate level are discussed. Element doping or substitution and group functionalization at the molecular level as well as control over morphologies, components, and dimensions at the aggregate level are reviewed to clarify structure/property relationships and further guide photocatalyst design. All the strategies discussed herein focus on enhancing hole and electron separation while suppressing their recombination, thereby improving the photocatalytic performance in evolution hydrogen. Finally, the key challenges and prospects of organic nanomaterials for photocatalytic evolution hydrogen are presented. We particularly focus on the construction of a system to evaluate the reasonable loading of co-catalysts, photocatalyst morphology regulatio...