The summer school sessions that colleges offer their undergraduates are sometimes considered supplementary activities and are rarely perceived as central to a college's mission or effectiveness. However, analyses of college transcript data that tracked a nationally-representative sample of undergraduates for several years and through multiple colleges show that those undergraduates who attend summer school at the end of their first year of college have better retention rates thereafter and are significantly more likely to complete a degree. This relationship remains statistically significant and of substantial size after controlling for student socio-demographic characteristics and for their academic performance prior to taking summer school, using propensity score matching methods. Moreover, students with lower academic performance and others at higher risk of dropping out who attend summer school also have higher graduation rates, suggesting that administrators should conceive of summer school enrollment as an important tool for improving undergraduate retention and degree completion.