Solar-to-hydrogen conversion is one of the most promising avenues to provide emission-free fuels and long-term chemical energy storage. Wide-range application crucially requires high-performance photocatalysts that are environmentally benign and free of precious metals. With high robustness, controllable composition, tunable electronic structure, and high porosity, 2D conjugated polymers (2DCPs) are among the most appealing candidates. We provide rational design principles for 2DCPs that are suitable for water splitting, highlight progress and open challenges in synthesis, and review the recent theory-based investigations on 2DCPs for photocatalysis. The role of theory as an indispensable tool is highlighted, as it promotes the understanding of the inner relationship between geometry, electronic structure, and photocatalytic performance of 2DCPs and, therefore, enables rational design of new photocatalysts.