Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) like molybdenum disulfide (MoS2) are generating significant excitement due to their unique electronic, chemical, and optical properties. Covalent chemical functionalization represents a critical tool for tuning the properties of TMDCs for use in many applications. However, the chemical inertness of semiconducting TMDCs has thus far hindered the robust chemical functionalization of these materials. Previous reports have required harsh chemical treatments or converting TMDCs into metallic phases prior to covalent attachment. Here, we demonstrate the direct covalent functionalization of the basal planes of unmodified semiconducting MoS2 using aryl diazonium salts without any pretreatments. Our approach preserves the semiconducting properties of MoS2, results in covalent C–S bonds, is applicable to MoS2 derived from a range of different synthesis methods, and enables a range of different functional groups to be tethered directly to the MoS2 surface. Using density functional theory calculations including van der Waals interactions and atomic-scale scanning probe microscopy studies, we demonstrate a novel reaction mechanism in which cooperative interactions enable the functionalization to propagate along the MoS2 basal plane. The flexibility of this covalent chemistry employing the diverse aryl diazonium salt family is further exploited to tether active proteins to MoS2, suggesting future biological applications and demonstrating its use as a versatile and powerful chemical platform for enhancing the utility of semiconducting TMDCs.