As a regionalism initiative, establishment of the ASEAN Community needs strong support from the public. However, ASEAN has been well-known for its state-centric, elitist, and non-existence of direct involvement from the people in ASEAN formation process. Previous researches have argued that any regional integration is influenced by the senses of identities among the people, but literatures show inconclusive findings on the effect of identities on supports. Thus, the study discusses the extents of regional and national identities and support for the ASEAN Community in 2015 among Gen Y public in four ASEAN countries, namely Malaysia, Indonesia, Thailand and Vietnam. It aims to investigate the interplay of these three variables among the public. Specifically, the study hypothesizes that, while senses of regional identity influence the support for regionalism initiative positively, nationalism sentiments may jeopardise the regionalism initiative because they theoretically go against the sense of regional identities and support. Using statistical analysis, the study tested these hypotheses by analysing and statistically tested the opinion among 1,471 Gen Y respondents in these four ASEAN countries surveyed in 2015 and found a conclusive finding on the contestation of regional identity and nationalist sentiments in the context of Southeast Asian regionalism.