This study tested whether the levels of coronavirus disease-2019 (COVID-19) severity, self-efficacy, knowledge, and preventive behaviors predicted mental health. Participants were 3190 Turkish adults (50% women; M age = 38.76, SD = 10.43 years) who completed online self-reported questionnaires. Most participants (55.11-64.42%) had inadequate knowledge about COVID-19 and were highly engaged in preventive behaviors. COVID-19-related severity, self-efficacy, and preventive behaviors correlated with mental health. Regression analysis showed that COVID-19 severity, self-efficacy, and preventive behaviors uniquely predicted mental health over and above gender, age, and chronic diseases. Findings may underscore the development of interventions aiming to improve mental health of individuals during pandemic.