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