Objectives: This cross-sectional online study examined the role of socio-demographic and psychological characteristics in predicting anxiety, stress, and concern of coronavirus (COVID-19) infection in older adults during the first and second waves of the COVID-19 pandemic in Slovakia. Methods: Six hundred and seven older adults (Sample 1) and 156 older adults (Sample 2) participated in the study during the first and second waves of the COVID-19 pandemic, respectively. The older adults completed a battery of validated instruments for anxiety, stress, concern of COVID-19 infection (outcome variables), satisfaction with health, perceived danger of COVID-19, loneliness, intolerance of uncertainty (IU), optimism, powerlessness, and coping self-efficacy (predictor variables). The data were analyzed using multiple linear regression. Results: The older adults in both samples experienced moderate levels of anxiety, stress, and concern of COVID-19 infection. The greatest amount of variability in anxiety and stress was explained by powerlessness, IU, optimism, and coping self-efficacy. Concern of COVID-19 infection was best predicted by the perceived danger of COVID-19. Conclusion: The findings contribute to the existing knowledge about the mental health of older adults during the COVID-19 pandemic and highlight the role of the psychological predictors of anxiety, stress, and concern of COVID-19 infection during both waves.