Vascular plants of the Kunashiri Island, the southernmost island of the Kuril Islands, were inventoried during an expedition to Kunashiri, 19–25 July 2013. This study was conducted as a part of the Sika Deer (Cervus nippon yezoensis) management program in Shiretoko World Natural Heritage Site, to assess vegetation damage under strong browsing pressure by the deer. A checklist of the vascular plants was made from each study locality. As Kunashiri Island is very close to Shiretoko Heritage Site, with similar climatic and geographical conditions, this plant list will provide a preliminary comparison of the floral composition between Kunashiri and Shiretoko or Hokkaido. Our research revealed that vegetation is well preserved on Kunashiri, and this floral list will serve as a reference of the original floral composition of Shiretoko World Natural Heritage Site prior to damage by deer.