Among US adults, the highest rates of hesitancy to receive the COVID-19 vaccine are among young adults aged 18 to 25. Vaccine hesitancy is particularly concerning among young adults in college, where social interactions on densely populated campuses can lead to substantial community spread. Given that many colleges have opted not to mandate vaccines, identification of modifiable predictors of vaccine hesitancy - such as perceived social norms - is key to informing interventions to promote vaccine uptake. To address this need, we examined predictors of and explicit reasons for vaccine hesitancy among 989 students aged 18 to 25 recruited from four geographically diverse US universities in the spring of 2021. At the time of the survey, 57.3% had been vaccinated, 13.7% intended to be vaccinated as soon as possible, and 29.0% were vaccine hesitant. Common reasons for hesitancy were wanting to see how it affected others first (75.2%), not believing it was necessary (30.0%), and other reasons (17.4%), which were examined via content analysis and revealed prominent safety concerns. Despite these varied explicit reasons, logistic regressions revealed that, when controlling for demographics and pandemic-related experiences, perceived descriptive and injunctive social norms for vaccine uptake were each significant predictors of vaccine hesitancy (ORs = 0.35 and 0.78, respectively). When both norms were entered into the same model, only perceived descriptive norms uniquely predicted vaccine hesitancy (OR = 0.37; 95% CI: 0.29 - 0.46). Findings suggest perceived social norms are strongly associated with vaccine-related behavior among young adult college students. Correcting normative misperceptions may be a promising approach to increase vaccine uptake and slow the spread of COVID-19 among young adults.