The relational turn in International Relations (IR) has made important contributions by challenging the substantialist claim to substance/thing as ontological primitives, by drawing much-needed attention to relations as ontologically fundamental, and by introducing a diversity of relational ways of being/becoming, knowing and doing. Yet, while rightly repudiating substantialism, the relational turn has remained ambivalent about the concept of substance itself, leaving open an important question: How should we understand substance within a relational ontology? As a result, we are left with different and sometimes confusing positions on the issue of substances vis-à-vis relations. Seeing this gap as a missed opportunity for relationalism in IR, this article seeks to bring substance back in without falling back into substantialism. It draws on a quantum conception of substance via the idea of quantum holography (QH) and its related notion of whole-part duality, and stresses the little-understood dual and inseparable nature of substance-relation (‘relatance’). The concept of substance-relation duality not only enriches our relational thinking, but also allows us to engage in relational analysis through a reimagined notion of substance. To illustrate, the article turns to a substance-based relational analysis of US-China relations.