The international legal order, although pluralist in structure, is in the process of being constitutionalized. This article supports this claim in several different ways. In the Part I, I argue that most accepted understandings of "constitution" would readily apply to at least some international regimes. In Part II, I discuss different notions of "constitutional pluralism," and demonstrate that legal pluralism is not necessarily antithetical to constitutionalism. In fact, one finds a great deal of constitutional pluralism within national legal orders in Europe. Part III puts forward an argument that the European Court of Justice, the European Court of Human Rights, and the Appellate Body of the World Trade Organization are constitutional jurisdictions. In the Conclusion, I respond what I take to be the most important objections to these claims.