Invasive species compete with native species and damage ecosystems. Due to their limited resources, island ecosystems are vulnerable to impacts of invasive species. In the Deokjeok archipelago, South Korea, invasive sika deer (Cervus nippon ssp. taiouanus) introduced for economic purposes are causing harm through severe browsing. This study aimed to evaluate long-term browsing impacts of invasive deer by tracking vegetation index changes from 1986 to 2020 with Landsat satellite imagery and the LandTrendr algorithm. We compared vegetation index trend using Sen’s slope and Disturbance/Recovery area ratio (D/R ratio) between Gureop-do, where these deer were introduced with rapid population increase, and Deokjeok-do, Baega-do, and Mungapdo where these deer have not been introduced yet. Results showed a decreasing trend of the vegetation index in Gureop-do, while other islands without those deer increased. The cumulative D/R ratio on Gureop-do was 212.44%, meaning that disturbance exceeded the recovery area more than two-fold. In contrast, the D/R ratios for other islands remained under 50%. Sen’s slope and t-test showed a significant decrease of NDVI in Gureop-do after deer introduction in 2000. By quantifying the browsing impact of invasive ungulates in island ecosystems using satellite imagery, time and costeffective strategies for invasive species monitoring are provided.