This research furthers our understanding of brand authenticity by conceptualising and measuring relational authenticity as brands being true in their relationships with consumers. We develop a scale through two purification stages and then assess the scale's predictive validity for various product and service brands. Sections of the Authenticity Inventory from psychology are adapted and subjected to exploratory and confirmatory factor analysis to confirm the dimensionality of relational authenticity. Discriminant and convergent validity tests confirm the four-item unidimensional construct is distinct from brand attachment. Next, we demonstrate the predictive validity of relational authenticity by identifying it as a stronger predictor of brand attitudes and purchase intentions than brand attachment. Relational authenticity increases brand attitudes and purchase intentions for those consumers with weak brand attachment. Finally, the predictive power of relational authenticity is further examined relative to two relationship quality measures of trust and commitment. Relational authenticity is a stronger predictor of brand attitudes than commitment to the brand and a stronger predictor of purchase intentions than trust in the brand. Marketers and brand managers can use relational authenticity as a basis for positioning or developing brands.