Deer can strongly affect other species as a keystone species. The intensity of their herbivory impact on forest vegetation is suggested to be affected by not only deer density but also by other environmental factors such as landscape components. Especially, artificial grasslands such as pastures and meadows possibility affect deer impact on forest vegetation by altering deer foraging behavior. Therefore, we aimed to clarify the effect of presence/absence of artificial grasslands on impact by sika deer (Cervus nippon) on forest vegetation. We compared the occurrence of debarking and the coverage of understory vegetation in two adjacent regions under similar deer density with and without artificial grasslands in Yamanashi Prefecture, Japan. The occurrence probability of debarking was more severe in forests in abundant artificial grassland region than in no artificial grasslands region despite considering the effects of deer density, tree size, and tree species. In contrast, the coverage of understory vegetation was high in abundant artificial grassland region than in no artificial grassland region despite considering the effects of light condition, vegetation type, and deer density. In conclusion, artificial grasslands modify deer impact on forest vegetation by increasing the occurrence probability of debarking but decreasing the browsing pressure on understory vegetation.