Having a diverse workforce in STEMM (science, technology, engineering, mathematics, and medicine) fields is essential for the United States to remain competitive in the global economy and to create a more just society. Faculty‐mentored undergraduate research experiences have been identified as a high‐impact practice for encouraging students from diverse backgrounds to pursue STEMM education and careers. Despite extensive research examining factors influencing the effectiveness of mentor−mentee relationships, there is still limited understanding of how differences or similarities in the social identities of mentors and mentees, which we term mentor−mentee discordance, impact undergraduates’ research experiences and outcomes. In this perspective, we propose that mentor−mentee discordance should be conceptualized as a multidimensional, continuous construct and suggest a global index to measure varying degrees of discordance in mentoring relationships. We also offer a conceptual model that incorporates the Discordance Index to systematize the understanding of the effects of discordant mentoring relationships on student development across social contexts and over time. Finally, we provide recommendations for future researchers, STEMM educators, and program directors who are interested in using the Discordance Index.