**Abstract** *Background*: When reopening universities in times of COVID-19, students still have to adhere to COVID-19 behavioral guidelines. We explored what behavioral determinants (and underlying beliefs) related to the adherence to guidelines are both relevant and changeable, as input for future interventions. *Methods*: A cross-sectional online survey was conducted (Oct–Nov 2020), identifying behavioral determinants (and underlying beliefs) of university students' adherence to COVID-19-guidelines, including keeping 1.5 m distance, getting tested, and isolating (N = 255). *Results*: Attitude, perceived norm, self-efficacy, and several beliefs (e.g., risk perception beliefs ‘I am not afraid because I am young’ [r = −0.33; p < .001]; attitudinal beliefs, e.g., ‘I feel responsible for telling people to adhere to guidelines’ [r = 0.37; p < .001]; self-efficacy beliefs, e.g., ‘COVID-19-prevention guidelines are difficult to adhere to’ [r = −0.30; p < .001]) were associated with intention to adhere to guidelines, and for those beliefs there was room for improvement, making them suitable as possible intervention targets. *Conclusions*: Students mostly adhere to COVID-19 guidelines, but there is room for improvement. Interventions need to enhance students' adherence behavior by targeting the most relevant determinants as identified in this study. Based on these findings, a small intervention was introduced targeting the determinants of students' adherence to guidelines.