In 2020, the UK formally departed the EU after nearly four years of intense manoeuvring and discussions since the referendum. In the post-hegemonic regional world order, Brexit meant a rejection of supranationalism. Despite the UK's leaving, the EU remains a cohesive regional group. ASEAN is an intergovernmental organisation in which each member state maintains complete sovereignty. ASEAN's regionalism has risen from strength to strength in the previous half-century, with a vision of uniting this varied area into one of peace and prosperity. The EU is ASEAN's second-largest trading partner, accounting for over a quarter of all FDI inflows. It works with Southeast Asia on a multilateral and bilateral basis, depending on the needs of individual member states. Its previous multilateral involvement with ASEAN was managed through the EU. ASEAN, on the other hand, works with other countries as dialogue partners, sectoral partners, and development partners. Now the question is how Britain will engage with ASEAN. Brexit is expected to have global economic, social, and political ramifications. The article focuses on the consequences and potential orientations of the United Kingdom's post-Brexit relationship with ASEAN.