Monolayer two-dimensional (2D) materials are of great interest because of their unique electronic structures, noticeable in-plane confinement effect, and exceptional catalytic properties. Here, we prepared 2D covalent networks of polyoxometalate clusters (CN-POM) featuring monolayer crystalline molecular sheets, formed by the covalent connection between tetragonally arranged POM clusters. The CN-POM shows a superior catalytic efficiency in the oxidation of benzyl alcohol, and the conversion rate is five times higher than that of the POM cluster units. Theoretical calculations show that in-plane electron delocalization of CN-POM contributes to easier electron transfer and increases catalytic efficiency. Moreover, the conductivity of the covalently interconnected molecular sheets was 46 times greater than that of individual POM clusters. The preparation of monolayer covalent network of POM clusters provides a strategy to synthesize advanced cluster-based 2D materials and a precise molecular model to investigate the electronic structure of crystalline covalent networks.