Two-dimensional (2D) stable CdS monolayer sheets are proposed using the state-of-the-art theoretical calculations. Three different conformers (planar, distorted, and buckled) are predicted which are separated by low energy barriers. These monolayer sheets are not only thermodynamically, mechanically, and dynamically stable but also can withstand temperature as high as 1000 K. Band edge alignment of these monolayer sheets and bulk CdS is done with respect to the water oxidation and reduction potential to evaluate their photocatalytic activities. Here we show a planar CdS monolayer sheet is the most promising material for visible light photocatalysis and can be used for electronic and optoelectronic devices. © 2016 American Chemical Society.