The lack of techniques for counter doping in two dimensional (2D) semiconductors has hindered the development of p/n junctions, which are the basic building blocks of electronic devices. In this work, low-energy argon ions are used to create sulfur vacancies and are subsequently “filled” with oxygen to create p-doped MoS2−xOx. The incorporation of oxygen into the MoS2 lattice and hence band-structure modification reveal the nature of the p-type doping. These changes are validated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy, and photoluminescence measurements combined with density functional theory calculations. Electrical measurements reveal a complete flip in carrier polarity from n-type to p-type, which is further examined using temperature-dependent transport measurements. The enhancement of p-field-effect transistor characteristics is facilitated by employing top-gated transistors and area-selective vacancy engineering only in the contact regions. Finally, on the same flake, an in-plane MoS2 (n)/MoS2−xOx (p) type-I (straddling) heterojunction with rectifying behavior and excellent broadband photoresponse is demonstrated and explained using band diagrams. The spatial/metallurgical abruptness (<100 nm) of the heterojunctions is ascertained using Raman mapping. This process of vacancy engineering, which enables air-stable, area-selective, controlled, CMOS-compatible doping of 2D semiconductors is envisioned to open new vistas in the development of high-performance electronic and optoelectronic devices. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim