Language policy and planning (LPP) has explicit and implicit dimensions.Different from explicit policies,implicit LPP is a loosely-defi ned concept encompassing notions like unplanned language planning,non-offi cial policy,covert policy,non-linguistic policy,and implicit goals of language policy.Taking the implicit LPP as a methodological perspective,this paper examines the English as a medium of instruction (EMI) planning activities in three Southeast Asian countries,namely Malaysia,Singapore and Thailand.The adoption of EMI policy in non-native English speaking countries tends to be delineated in offi cial discourse as a language acquisition planning endeavour in order to improve students’ English language profi ciency and serve the economic good in the milieu of globalization.However,part of the EMI policy,particularly its political intentions,is left unstated and the de facto policy in practice is often somewhat disparate from the explicit policy.This paper is intended to reveal the nature of implicit EMI policies in Southeast Asia comprehensively.First,existing studies of implicit LPP are reviewed and the main constructs are generalized.Then the EMI policies in Malaysia,Singapore and Thailand are explored to unravel the complex interactions among the wide range of social,cultural and historical contexts,policy texts and agency of actors involved.This implicit LPP study of Southeast Asian countries may deepen our understanding of the local cultures and the promotion of harmony among diverse cultures,and in the meantime,contribute to the realization of the national vision of “the Belt and Road Initiatives” being implemented in China.