The reaction within Southeast Asia from business and policy leaders to the United Kingdom’s decision to exit the European Union has thus far been one of disappointment rather than panic. How Brexit will affect a possible EU–ASEAN free trade agreement, bilateral trade deals in the region, foreign direct investment flows between ASEAN and the U.K. and the EU, and the U.K.’s future trade arrangements remains uncertain. This article examines how the U.K.’s departure from the Union will affect ASEAN and its member states in terms of European trade, and its impact on the U.K.’s and the EU’s trade relations with Southeast Asia. It is argued that ASEAN is far from being immune to the effects of Brexit on financial markets and trade configurations. However, given the diversity of trade and investment among the ASEAN member states, aggregated regional effects and member-state effects will differ. The findings also suggest that there will be political and security considerations for ASEAN and its member states whilst the U.K. tries to establish itself as an independent trade actor post Brexit.