Background. This study is based on self-determination theory and the research on dispositional optimism and unrealistic optimism. Dispositional optimism is known to be protective of well-being and is related to adaptive coping strategies. Investigations related to unrealistic optimism, on the other hand, revealed that it may have both positive and negative consequences. Objective. To investigate dispositional optimism and two kinds of specific optimism as predictors of autonomous motivation to follow stay-at-home orders during the COVID-19 pandemic in a sample of Russian young adults: constructive optimism, meaning belief in the role of effort; and defensive optimism, meaning unrealistic expectations and denial that a problem exists. Design. A correlational (cross-sectional) study was conducted to measure adherence to the recommendation to stay at home, autonomous motivation, dispositional optimism, constructive optimism, and defensive optimism. An online survey was completed by 1,403 young adults (68% women) during the first month of lockdown. Results. The findings demonstrate that constructive optimism and its underlying dispositional optimism predict both autonomous motivation and adherence to the recommendation to stay at home, while defensive optimism produces the opposite, undermining effects. Structural equation modeling revealed the effect of gender on adherence to the recommendation (higher in women), mediated by different types of optimism and autonomous motivation. Conclusion. Dispositional optimism together with situation-specific constructive and defensive types of optimism are essential for explaining the health-related behavior and its motivation. These results contribute to self-determination theory, considering the role of personality factors in determining motivation.