This chapter examines the nature of Thailand’s foreign relations with Southeast Asia, particularly its neighbouring countries and the Association of Southeast Asian Nations (ASEAN) as a group. It demonstrates that Thailand’s foreign policy posture towards this region exhibits an aspiration to maintain a leading role and status. When scrutinising in detail, however, it can be seen that mainland Southeast Asia is the core of Thailand’s foreign policy formulation and ASEAN is deemed to be rather an extension of this policy. Undeniably, ASEAN has become an important platform for Thai foreign policy since the conflict in Cambodia throughout the 1980s and, particularly, in the post-Cold War era. However, the focus on mainland subregion remains intact. Therefore, Thailand’s foreign policy towards Southeast Asia in the post–Cold War period is criticised on many occasions that it bypasses ASEAN.