Organic semiconductors (OSCs) have the advantages of tunable molecular structures, suitable band gaps, and exceptional optoelectronic properties. The π–π stacking ability of OSCs also leads to appealing molecular stacking structure, function, and stability. So far, organic photocatalysts have engaged in homogeneous or heterogeneous photocatalysis in the form of free molecules, supported molecules, or nanostructures. Meanwhile, researches on organic photocatalysts have expanded from small organic molecules to the organic macromolecules, as well as their various nanostructures and nanocomposites including isolated zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), three-dimensional (3D) nanostructures, and their combinations. Therefore, many versatile strategies have been explored to improve photocatalytic ability and practicality either from molecular synthetic modification, crystal, or interface engineering. In this review, we first discuss the photophysical and photochemical processes of organic photocatalysts that govern the ultimate photocatalytic efficiency; we then summarize different forms of organic photocatalysts, their rational design strategies, and mechanistic pathways, as well as their applications in H₂ evolution, CO₂ reduction, and environmental purification, aiming to highlight the structure/property relationships; we lastly propose ongoing directions and challenges for future development of organic photocatalysts in real use.