Article 38(1)(d) of the Statute of the International Court of Justice attributes limited legal authority to judicial and arbitral decisions. They are not formal sources of law and are described as only subsidiary means for the determination of rules of law. However, the continuing validity of this characterization is challenged not only by the Court’s practice of referring to its own jurisprudence, a phenomenon that has been empirically and theoretically analysed elsewhere, but also its relatively new practice of relying on external case law. This article seeks to draw attention to one aspect of this new practice, namely the marked increase in the Court’s citation of inter-state arbitral awards since the 1990s. It is argued that the Court refers to inter-state arbitral awards in its decisions for three principal reasons – (i) to determine the existence of a given rule, (ii) to supplement its legal reasoning or its own case law on a particular issue, and (iii) to distinguish an arbitral award from the case before it. More ambitiously, the article argues that the way the Court relies on inter-state arbitral awards shows that the Court attributes legal authority to these awards that goes beyond that of a subsidiary means for determining a given rule of law, bringing it closer to what might be qualified as persuasive but non-binding precedent.