The traditional division of Southeast Asian legal systems into anglophone common law jurisdictions and non-anglophone civil law systems has lost some of its validity in recent decades. On the one hand, postcolonial language planning has brought a much greater role for local languages in several common law systems. On the other hand, English is becoming an important part of legal drafting and professional training in several civil law systems as the influence of economic globalisation and cross-border legislation increases. Using evidence from courtroom observations and interviews with lawyers, this article considers the extent to which English is becoming an instrument of legal harmonisation around the region and how attitudes to the language reflect tensions between convergent legal practices and divergent legal cultures.