Objectives Due to an increased infection rate among young adults, they need to adhere to the preventive guidelines to stop the spread of COVID-19 and protect vulnerable others. The purpose of this mixed methods study was to explore the role of risk perception and affective response in the preventive behaviours of young adults during the COVID-19 outbreak. Setting This study followed a convergent mixed methods design, in which a quantitative online survey (n=1081) and 10 qualitative in-depth semistructured video interviews were conducted separately in the Netherlands during April–August 2020. Participants 1081 participants filled in the online survey, and 10 participants participated in the interviews. Eligibility criteria included being a university student. Primary and secondary outcome measures Data on risk perception, affective response, that is, worry, and adherence to preventive guidelines were combined and analysed during this study. There were no secondary outcome measures. Results The results showed that young adults perceived their risk as low. Their affective response for their own well-being was also low; however, their affective response was high with regards to vulnerable others in their surroundings. Due to their high impersonal risk perception (ie, perceived risk to others) and high affective response, young adults adhered to most preventive guidelines relatively frequently. However, young adults sometimes neglected social distancing due to the negative effects on mental health and the uncertainty of the duration of the situation. Conclusions In conclusion, high impersonal risk perception and high affective response regarding others are key motivators in young adults’ preventive behaviour. To maximise adherence to the preventive guidelines, risk communication should put emphasis on the benefits to vulnerable others’ health when young adults adhere to the preventive guidelines.