The Greater Mekong Sub-Region (GMS) is regarded as a single tourism market. This paper investigates international tourist visits to the GMS. We employ the panel data approach by using data from the GMS countries and their four major source countries i.e. Japan, Malaysia, China and Korea. The study concludes that capital investment is the most important factor for international tourism supply while income level is the most important factor for international tourism demand. Hence, we suggest that GMS countries should offer tourists a unique experience by presenting nature tourism and ecotourism. They can focus on attracting higher-income international tourist groups to improve their tourism markets and increase their competitiveness in tourism in order to achieve a sustainable single tourism market.