The objective of this research was to examine the relationships of the five key foreign tourist markets in the Greater Mekong Sub-region (GMS), Cambodia, Laos, Myanmar, Thailand, and Vietnam. Quarterly data of tourist numbers from China, Japan, Korea, Malaysia, and the USA who traveled to the countries in GMS during the period 2001-2017 were used. Vector Autoregressive (VARs) was used to test Granger Causality to explain the relationships among tourist markets in the region. The findings revealed that tourist markets in Cambodia had forward relations with Laos and Vietnam. Those in Laos had forward relationship with Myanmar and Thailand. Tourist markets in Myanmar had forward relationship with Vietnam, Laos, and Thailand. Tourist markets in Thailand had forward relationship with Vietnam, Laos, and Myanmar whereas those of Vietnam had forward relationship with Laos, Myanmar, Thailand, and Cambodia. The overview shows that the key tourist markets in each country supported each other.