Surfaces play a key role in determining material properties, and their importance is further magnified in the two-dimensional (2D) limit. Though monolayers are entirely composed of surfaces, there is no chemical approach to covalently address them without breaking intralayer bond. Here, we describe a 2D semiconductor that offers two unique features among 2D materials: structural hierarchy within the monolayer and surface reactive sites that enable functionalization. The 2D semiconductor is composed of a single layer of strongly interconnected Re6Se8 clusters arranged in an oblique lattice capped by substitutionally labile Cl atoms. We show that a simple ligand substitution strategy borrowed from traditional coordination chemistry can be used to modify the surface of the 2D material while preserving its internal structure. The potential generality of this approach establishes a promising route toward multifunctional 2D materials with tunable physical and chemical properties and may also facilitate better electrical top contact to 2D semiconductors.