The figure of amicus curiae is a central feature of contemporary investor-state arbitration law and practice. Over the last several years the European Commission has taken part in a number of arbitral proceedings that touched upon matters of European Union Law. This phenomenon is part of the European Union’s broader incursion into the realm of investment law. The participation of an entity with legislative and political functions in investment arbitrations raises complex questions regarding the nature of the interests that this entity pursues and the potential impact that its involvement might have in the dispute settlement mechanism. This paper examines the participation of the European Commission (EC) in investor-state arbitrations and assesses its impact in the overall mechanism of investor-state dispute resolution. It is argued that the EC is fundamentally a distinct type of amicus, as it pursues interests different from those of traditional amici, and should therefore be accorded extended participatory rights.