The US Supreme Court is regarded by liberal political philosophers, such as John Rawls, as an exemplary institution of public reason, since its task is to maintain constitutional values. Public reason is the reason of the public of a constitutional state; its content is given by shared, stable constitutional values all citizens are expected to endorse. If the reason of the US Supreme Court is public in the above sense, the court contributes to the legitimacy of US law and politics. Since the EU is, unlike the US, a regional economic union, based on a constitutional treaty that every now and then adapts to new political circumstances, this paper asks: is the notion of public reason relevant in the case of the EU and, if so, can the ECJ, as an exemplary institution of the EU’s public reason, contribute to the legitimacy of the EU and, consequently, of itself?