This article aims to review the evolution of the Republic of Vietnam (RVN)’s involvement in the Association of Southeast Asian Nations (ASEAN), which was founded in 1967 by Thailand, Malaysia, Singapore, the Philippines, and Indonesia. South Vietnamese leaders and diplomats designed a new foreign policy under the administration of Nguyen Van Thieu (1967–75) that shifted focus to Southeast Asia alongside the RVN’s long-standing camaraderie with the United States. This demonstrated Thieu’s keenness to engage with regional states for the purpose of nation-building and an anti-Communist future. The RVN’s engagement with ASEAN reflected its efforts to foster a regionalization process along with peace, stability, and development in Southeast Asia, particularly after the withdrawal of the US and its allies. This study uses a qualitative approach, employing a wide range of archival collections housed at the National Archives Center II, Ho Chi Minh City and a handful of desk-research papers. The relationship is periodized into two phases. During the first phase (1967–72), the RVN embraced ASEAN’s values and, despite its observer status, expected help in achieving its security and economic goals. After the 1973 Paris Peace Accords, ASEAN increasingly distanced itself from the RVN as members changed their stances, particularly as attacks by the Democratic Republic of Vietnam escalated in RVN territories. This paper aims to bridge a gap in scholarship by examining the positioning of the RVN in international and regional relations during the Cold War.