Background: This study investigated the effects of stress events (reduced income, change of jobs, family conflicts, and conflicts with acquaintances) experienced in the prolonged COVID-19 situation on psychological distress (peritraumatic distress, depression, and loneliness). It also tested the moderating effects of social support between them according to the stress-buffering model. Methods: Data was collected from 800 adult men and women, including early adulthood (ages 19∼30), early middle-aged adulthood (ages 31∼45), late middle-aged adulthood (ages 46∼64), and late adulthood (ages 65 and older), via an online survey covering peritraumatic distress, loneliness, depression, and social support. Hierarchical regression analysis was conducted to test the moderating effects of social support. Results: First, reduced income, change of jobs, and interpersonal conflicts experienced during the COVID-19 pandemic were positively associated with peritraumatic distress, depression, and loneliness. However, social support was negatively associated with them. Second, social support had no moderating effects on the impacts of change of jobs and reduced income on peritraumatic distress, depression, and loneliness. It had moderating effects only on the influences of interpersonal conflicts. Finally, the study examined the moderating effects of social support by the age group and found that it mitigated the impacts of interpersonal conflicts in all of the age groups except for the early middle-aged group. Conclusions: These findings demonstrate that social support can mitigate psychological distress due to interpersonal stress in a pandemic situation and imply a need for differentiated intervention according to the different stress-buffering effects of social support according to age groups.