Since COVID-19 currently has no proven cure but high morbidity and mortality, many people are living in fear of the virus along with other mental health challenges induced by the lockdowns and social distancing. Hence, this study aims to provide evidence on the co-occurrence and inter-relations between the fear of COVID-19, post-traumatic stress symptoms, and psychological distress in adherence to preventive health behavior among Nigerians. It also seeks to determine whether this process differs for men and women. The sample comprised 1,172 consenting young adults (mean age 5 22.9 6 6.6 years, 54.5% females) selected using a snowball sampling technique. Structural equation modeling (SEM) was used to test the mediation model of post-traumatic stress symptoms and psychological distress as parallel and serial mediators of the relationship between the fear of COVID-19 and preventive health behavior. The indirect effect of the fear of COVID-19 on preventive health behavior across gender was tested using moderation analysis. Results showed that post-traumatic stress symptoms and psychological distress serially and fully, in causal order, mediated the association between the fear of COVID-19 and preventive health behavior, and gender moderated the mediation effects. The research provides evidence that the fear of COVID-19 could trigger preventive health behavior through post-traumatic stress symptoms but reduces it through psychological distress, whereas the fear of COVID-19 has a slightly more positive impact on preventive health behavior among men.