Abstract Heterostructures between 2D and 3D electron systems remain critically important in developing novel and efficient optoelectronic and electronic devices. In this study, a vertical heterojunction between monolayer MoS2 and bulk InSe was developed. This heterojunction exhibits a type-I band alignment that facilitates rapid energy transfer from the wide bandgap MoS2 to the narrow bandgap InSe resulting in quenching of the MoS2 photoluminescence (PL) emission and enhancement of the A exciton emission in InSe. Temperature-dependent PL measurements of MoS2 on SiO2, MoS2 on InSe, and bare InSe revealed the critical role of defect trapping and electron-phonon coupling in the optical response of MoS2 on InSe. These results demonstrate that heterostructures combining monolayer MoS2 on bulk InSe, showing marked improvement relative to bare InSe, would be advantageous when incorporated into optoelectronic devices such as photodetectors, light emitters, and color converters and highlights the benefit of creating van der Waals (vdW) heterostructures with tailored properties.