In the face of public contestation, international organizations (IOs) invoke norms in their public communication to enhance relevant audiences' legitimacy beliefs. This article offers the first comprehensive analysis of what we term normative diversity in IOs' discursive legitimation by drawing on a novel dataset on norm-based justifications in more than 32,000 paragraphs of text published by 28 regional IOs between 1980 and 2019. We show that IOs vary strikingly in this respect: whereas some IOs invoke a narrow set of norms, often focused on economic welfare and functional capability, others engage a wide variety that includes security, national sovereignty, democracy or human rights. To explain this variance, we specify and test an explanatory framework that emphasizes IO audiences, agents and peer organizations as distinct origins. Our statistical analysis reveals that IOs diversify their discursive legitimation to 1) address heterogeneous audiences, 2) reconcile competing beliefs amongst agents themselves, and 3) integrate the legitimation of peer IOs. These findings indicate that IOs respond to the growing complexity of international cooperation in their discursive legitimation and may raise policy-makers' awareness of the difficulties in contemporary legitimation efforts.