Monolayer transition metal dichalcogenides (TMDs) have been considered as promising materials for various next-generation semiconductor devices. However, carrier doping techniques for TMDs, which are important for device fabrication, have not been completely established yet. Here, we report a monolayer p–n junction formed using in situ substitutional doping during chemical vapor deposition (CVD). We synthesized monolayer MoS2–Nb-doped MoS2 lateral homojunctions using CVD and then characterized their physical and electrical properties. The optimized growth condition enabled us to obtain spatially selective and heavy Nb doping in the edge region of a single-crystalline MoS2, thus resulting in an obvious work function difference between the inner and edge regions of the crystal. The obtained monolayer crystal demonstrated n-type and degenerate p-type semiconducting behaviors in each region, and a clear rectifying behavior across the n-type and p-type interface was observed. We believe that the results obtained can expand the research field of exploring two-dimensional homo p–n junctions, which can be important for realizing various TMD-based devices, such as diodes and field-effect transistors, with low-contact resistance.