Background and Aim: The coronavirus pandemic has had a negative impact on the health and well-being of communities and individuals around the world. One of the concerns about the COVID-19 epidemic is the impact of social isolation due to social distancing on the mental health of older adults. This study aimed to investigate the relationship between mental health and loneliness among seniors during the COVID-19 pandemic. Materials and Methods: In this cross-sectional study, 211 retirees of Kermanshah University of Medical Sciences were selected by available sampling method in 2021. Data were collected through the Demographic, Mental Health Questionnaire (GHQ-28) and the Adult Social and Emotional Feeling Questionnaire (SELSA-S). Data were analyzed using paired t
-test, one-way analysis of variance, covariance, and chi-squared tests. The significance level was considered 0.05. Results: The mean age of participants was 71.26 years and the results showed a significant relationship between feeling lonely and mental health (r = 0.535, p
= 0.001). There were statistically significant relationships between mental health and social functioning (r = −0.204, p
≥ 0.001) and depression (r = 0.501, p
= 0.001) and also between mental health and three subscales of loneliness: romantic loneliness (r = 0.001, p
= 0.001), social loneliness (r = 0.493, p
= 0.001), and family loneliness (r = 0.289, p
= 0.001). Conclusion: Feeling lonely during the coronavirus disease pandemic has a negative effect on the mental health of older adults; therefore, it is necessary for mental health professionals to improve the mental health of the elderly with proper planning and interventions.