Levels of civic engagement are assumed to vary according to numerous social and psychological characteristics, but not much is known about online civic engagement. This study aimed to investigate differences and similarities in young people’s offline and online civic engagement and to clarify, based on Ajzen’s theory of planned behavior (TPB), associations between motivation for civic engagement, peer and parental norms, collective efficacy, and civic engagement. The sample consisted of 755 youth (native German, ethnic German Diaspora, and Turkish migrants) from two age groups (16–18 and 19–26; mean age 20.5 years; 52 % female). Results showed that ethnic group membership and age moderated the frequency of engagement behavior, with Turkish migrants taking part more than native Germans, who were followed by ethnic German Diaspora migrants. Analyses based on TPB showed good fit for a model relating intention for offline and online civic engagement to motivation for civic engagement, peer and parental norms, and collective efficacy. Ethnic group moderated the findings for offline civic engagement and questioned the universality of some model parameters (e.g., peer and parental norms). This study showed the utility of the TPB framework for studying civic engagement but also reveals that the predictive utility of peer and parental norms seems to vary depending on the group and the behavior under study. This study highlights the importance of including minority samples in the study of civic engagement in order to identify between-group similarities and differences.