Science, technology, engineering, and mathematics (STEM) undergraduate researchers (UGRs) work under the direction of faculty mentors. Many also work with postgraduate mentors, either graduate students or postdoctoral personnel, but little is known about how these mentors shape student experiences. Using original survey data from 516 participants across 78 National Science Foundation Research Experiences for Undergraduates Sites in Summer 2022, we find that 44% of students had a postgraduate mentor. Our multivariable multilevel analyses of UGRs with postgraduate mentors (n = 229) revealed that having a different gender identity than one's postgraduate mentor, having a competent postgraduate mentor, having a graduate student mentor, and spending more than 26 weekly hours with a postgraduate mentor were associated with greater science identity, personal, and intellectual gains. Matching with one's postgraduate mentor based on racial identity was associated with greater science identity gains. We also find that Hispanic and multiracial/other race students gained more compared with their White peers, and transgender and gender nonconforming students gained less compared with their men peers. Because the attributes and behaviors of postgraduate mentors are integral to undergraduate research experiences, analysts and practitioners should treat postgraduate mentorship as an essential part of the undergraduate research enterprise.