Since the entry into force of the Lisbon Treaty, the Member States of the European Union (EU) remain pivotal actors in international investment law despite transferring foreign direct investment (FDI) to the EU’s exclusive competence. This contribution highlights their role in shaping EU investment protection policy through treaty-making. The main argument it puts forward is that the Member States’ engagement in the “making” and “unmaking” of international investment agreements (IIA) has transformative effects on the EU as an international actor. In examining the domestic institutional framework within which the Member States exercise their treaty-making powers, we will demonstrate how far national institutions—legislature, executive, and judiciary—influence the EU’s investment policy and treaty practice from the bottom up. The analysis posits that the parallel treaty-making of the Member States and the EU may help to ensure that the EU investment policy and treaty practice is inspired and legitimised by the national base but perplexes, at the same time, the Union’s capacity to act coherently in international investment law, a symptom of the competence conundrum under EU law.