Heterostructures of compositionally and electronically variant two-dimensional (2D) atomic layers are viable building blocks for ultrathin optoelectronic devices. We show that the composition of interfacial transition region between semiconducting WSe<sub>2sub> atomic layer channels and metallic NbSe<sub>2sub> contact layers can be engineered through interfacial doping with Nb atoms. W<sub>xsub>Nb<sub>1–xsub>Se<sub>2sub> interfacial regions considerably lower the potential barrier height of the junction, significantly improving the performance of the corresponding WSe<sub>2sub>-based field-effect transistor devices. The creation of such alloyed 2D junctions between dissimilar atomic layer domains could be the most important factor in controlling the electronic properties of 2D junctions and the design and fabrication of 2D atomic layer devices.