We report on an optically probed ferromagnetic resonance experiment which elucidates 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, . 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.