In the sub-1 nm scale region, nanomaterials have shown some novel and interesting chemical and physical properties, such as flexibility, polymer-like rheology, etc. However, how to limit the size of hybrid nanomaterials into sub-1 nm scale is still a great challenge. Herein, by incorporating phosphomolybdic acid (PMA) clusters into inorganic materials (CuO) during the nucleation step, we first synthesized the twodimensional (2D) hybrid CuO-PMA sub-1 nm nanosheets (SNSs). Molecular dynamics (MD) simulations demonstrated that PMA clusters interacted with CuO molecules and coassembled into stable 2D simple square SNSs. The CuO-PMA SNSs not only performed well photothermal conversion property at 808 nm laser, but also could be applied in solar steam generation with an energy conversion efficiency of 95.72% under 1 sun illumination. This work would provide a promising avenue for the synthesis and wider applications of cluster−inorganic materials-based 2D hybrid SNSs.