The two-dimensional (2D) heterojunctions of layered transition metal dichalcogenides (TMDs) with different bandgaps are the basis of modern electronic and optoelectronic devices such as high-speed transistors, light-emitting diodes, diode lasers and so on. Although, complex heterostructures (HSs) have been widely fabricated in the vertical direction via van der Waals (vdWs) stacking of different TMDs, but, atomic stitching of such 2D materials in the horizontal direction is proven to be so far most challenging. Here, we report a two-step sequential growth of monolayer n-type MoSe2 - p-type WSe2 lateral junction using chemical vapor deposition (CVD), which was confirmed from Raman and photoluminescence measurements. This work could be extended to other families of TMDs and provide a platform for the development of new device functionalities such as in-plane transistors and diodes to be integrated within a single atomically thin layer. © 2020 American Institute of Physics Inc.. All rights reserved.