A new pattern of bilateralism is evident in Southeast Asian economic diplomacy, and this may be broadly viewed from extra-regional and intra-regional perspectives. Regarding the former, an increasing number of states from the Association of Southeast Asian Nations (ASEAN) group have engaged in the Asia-Pacific's new bilateral free trade agreement project trend, and two ASEAN member states - Singapore and Thailand - have been at its forefront. Regarding the latter dimension, recent developments in intra-ASEAN diplomacy have revealed the emergence of a Singapore-Thailand bilateral axis or alliance on matters of Southeast Asian economic regionalism. These two dimensions of economic bilateralism are studied in relation to their implications for Southeast Asian or ASEAN-led regionalism. In this context, region-convergent bilateralism can make positive contributions to the development of regionalism, whereas region-divergent bilateralism essentially undermines regional community-building endeavours. This forms the conceptual framework for studying the impact of Singapore and Thailand's active bilateral economic diplomacy upon ASEAN's own regional economic projects, such as the ASEAN Free Trade Area (AFTA), and also on ASEAN as an organization for fostering Southeast Asian economic regionalism generally. It is contended that based on both the deeper strategic intentions behind Singapore's and Thailand's foreign economic policies and wider international political economy considerations the region-divergent outcomes are more likely to arise within Southeast Asia from the economic bilateralism they are currently championing.