This study examines how retirees' formal and informal social participation change over time and investigates gendered differences. Seven waves of the Korean Longitudinal Study of Ageing track the frequency of social participation. A comparison group of employed individuals, weighted with coarsened exact matching, controls for age and time trends, and mixed model regressions estimate changes over time. Retirees show a gradual decline in the frequency of meeting friends and an abrupt decrease in the frequency of attending a social gathering, compared to their working peers. These trends are much stronger for men than women, and compound pre-existing gender differences in social participation. The more severe decline in the social participation of men is of great concern and points to the persistence of gendered employment structures into the retirement transition, putting Korean retirees at risk of social isolation and related health deterioration.