Purpose: Recent research on graduate students' diversity, equity and inclusion (DEI) socialization found that graduate colleges play a role in supporting graduate students' DEI professional development (Perez et al., 2020), but more studies are needed about how graduate colleges facilitate DEI socialization. One graduate college at a large, selective, research-intensive, public university in the Midwestern US created a graduate certificate for professional development in DEI to expand graduate students' capacities to contribute to inclusion and equity in higher education. The purpose of this multi-method program evaluation is to assess whether the certificate program created significant learning about DEI and developed intercultural competence among graduate students. Design/methodology/approach: The authors rely on multiple methods to evaluate the impact of the professional development DEI certificate. First, the authors used the Intercultural Development Inventory (IDI) pre and postassessment to measure the growth of participants in the first three years of the program. Second, the authors designed a reflection tool to assess significant learning after each component of the program. Finally, we conducted focus groups with graduates of the program to understand what program components were most valuable for DEI-related significant learning. Findings: The authors found that the DEI professional development program increased students' intercultural competence as measured by the IDI. Students reported perceptions of significant learning in every domain of learning we assessed using a self-reflection tool and in focus groups. Originality/value: To the best of the authors' knowledge, this is the first study that demonstrates how graduate colleges contribute to DEI socialization by preparing graduate students to interact across differences and contribute to inclusive climates both within and beyond academe.