Racial and ethnic disparities in STEM achievement are associated with weaker economic growth, greater social inequalities, and narrower parameters of scientific inquiry. Extant research suggests that undergraduate research experiences (URE) can reduce those disparities by enhancing perceptions of belonging and scientific self-efficacy among students from underrepresented groups. However, to date, very few studies have examined the relationship between URE and "post-baccalaureate" educational achievement gains among such students and those that have tend to be limited in terms of causal leverage and generalizability. In this study, we aim to make progress by analyzing data from the California State University system's longstanding Louis Stokes Alliance for Minority Participation (CSU-LSAMP) program. Applying a quasi-experimental research design and drawing upon a large and representative sample of students whom we tracked over time, we observe that URE is strongly associated with post-baccalaureate enrollment and graduation in STEM disciplines among students from underrepresented backgrounds.