Japan is one of the most important partners of Thailand. The Japanese government has been contributing to the Thai economy and industrial development in many ways. The ASEAN and the Japanese government work closely to support the developmental arrangements, both hard and soft infrastructures for regional connectivity. The ASEAN Connectivity is one of the most significant schemes that has been playing an important role in helping Thailand to link with its neighbours. The Japanese government helped ASEAN member countries to obtain financial and technical assistance to ASEAN Connectivity development projects. However, the engagement and contribution of Japan have recently competed with China. ASEAN Connectivity, thus, is one of the possible areas of political rivalries. This paper aims to analyse two main questions. Firstly, how has the Thai government developed its strategy and position to balance its relations with Japan and China? Secondly, how has ASEAN Connectivity, particularly Japan-funded development projects along the East-West Economic Corridor (EWEC) and Southern Economic Corridor (SEC), induced any economic and political development in Thailand?