This research article shed light on the Republic of Vietnam (the RVN) – Thailand diplomatic relation over the course of the Vietnam War (1965-1973). The author employed historical approach, along with qualitative methods, and stated that the RVN and Thailand put emphasis on the mutual concern about the insurgencies of Communists and closely linked with the US to deter Communismin Southeast Asia. Also, the RVN attempted to approach overseas Vietnamese Thai, performing its duty toward the heart and minds of those groups. Despite the severe war, the RVN initially forged economic links and public diplomacy with Thailand to promote international economic relations in the region.