The public policy exception is a well-established part of common law choice of law doctrine. But to use the exception, judges must confront a difficult conceptual issue: they must identify the forum’s fundamental values that cannot be violated by foreign law. Private international law scholars have long argued that it is impossible to provide a compelling account of the exception’s juridical role and the norms that should govern its use. In this paper, I argue that common law constitutionalism can offer a compelling account of the exception. Drawing on the work of David Dyzenhaus and TRS Allan, I argue that the exception is a means by which common law courts can ensure that foreign law does not violate the fundamental constitutional requirements of the common law. This offers a new way of understanding the role that public policy plays in choice of law and the values that should inform its use. It can also make sense of leading public policy jurisprudence, such as Somerset’s case.