Excessive use of fossil fuels leads to energy shortages and environmental pollution, threatening human health and social development. As a clean, green, and sustainable technology, generation of renewable energy from solar light through photocatalysis has received increasing attention to cope with the impending energy and environmental crisis. The atomically thin two-dimensional (2D) semiconductors with large surface area and abundant low-coordinate surface atoms prove to exhibit enormous potential to attain efficient photocatalytic performance. These 2D ultrathin materials can shorten the transport distance of charge carriers from the interior to the surface, enhance reactants' (e.g. CO2 and H2O) adsorption and activation to lower the energy barrier, promote specific reaction processes and inhibit competitive reactions, and regulate the efficiency and selectivity of the catalytic reaction. This Feature article provides a concise overview of the preparation, catalytic mechanism, strategies for boosting the photoconversion performance, various photocatalytic applications, and characterization techniques of atomically thin 2D semiconductors. The major challenges and opportunities of the ultrathin photocatalysts are also addressed. It is hoped that this review can provide useful guidelines toward further research on 2D nanocatalysts, and inspire practical applications of these unique materials for energy conversion.