  Spin and photonic systems are at the heart of modern information devices and
emerging quantum technologies. An interplay between electron-hole pairs
(excitons) in semiconductors and collective spin excitations (magnons) in
magnetic crystals would bridge these heterogeneous systems, leveraging their
individual assets in novel interconnected devices. Here, we report the
magnon-exciton coupling at the interface between a magnetic thin film and an
atomically-thin semiconductor. Our approach allies the long-lived magnons
hosted in a film of yttrium iron garnet (YIG) to strongly-bound excitons in a
flake of a transition metal dichalcogenide, MoSe$_2$. The magnons induce on the
excitons a dynamical valley Zeeman effect ruled by interfacial exchange
interactions. This nascent class of hybrid system suggests new opportunities
for information transduction between microwave and optical regions.
