Wearing a face mask is effective in minimizing the spread of coronavirus disease 2019 (COVID-19) among unvaccinated individuals and preventing severe illness among the vaccinated. Country, state and local guidelines promote, and at times mandate, mask-wearing despite it being publicly perceived as an individual's choice. Guided by the Health Belief Model (HBM), structural equation modeling was used to analyze longitudinal data in a sample of US adults aged 18-49 years to identify constructs that contribute to face mask-wearing. Results indicated that perceived COVID-19 severity, perceived masking benefits and self-efficacy were positively associated with masking behavior, and masking barriers were negatively associated with masking behavior. Perceived susceptibility to COVID-19 and cues to action were nonsignificant correlates of masking behavior. These results' theoretical and practical implications contribute to the literature on the HBM and the COVID-19 pandemic. Future directions and limitations are discussed.