This chapter discusses the role that border tourism plays in enhancing the sustainability of a border island by focusing on Tsushima Island in Nagasaki Prefecture. Located between Kyushu, Japan and Busan, South Korea, Tsushima has played an important role as a front line both of national defense and international exchange between Japan and the Korean Peninsula since ancient times. There have been no territorial disputes except for a period in the Middle Ages, and in intergovernmental relations sovereignty is clearly delineated, making it the most legally stable borderland in Japan. Therefore, the traffic of people is active, and in 2019, more than 400,000 foreign tourists visited Tsushima annually. Most of them are Koreans, but for the sustainability of Tsushima, which is located in the border region of Japan territory and has a declining population and industry, the acceptance of Korean tourists is one of the lifelines. On the other hands, Japanese tourists do not usually cross the border between Tsushima and Busan and it is expected to develop such a cross-border tourism by Japanese tourists. Recently, efforts to enhance sustainability by developing border tourism have faced serious challenges; deterioration of the Japan-South Korea diplomatic relations, the COVID-19 pandemic and a temptation to receive subsidies from national government by accepting nuclear waste. Based on the results of fieldwork and interviews conducted in Tsushima for more than 10 years, this chapter examines the possibility and significance of border tourism for enhancing the sustainability of Tsushima island.