Aim Social activity seems to be important for the prevention of cognitive impairment and frailty. The objective of the present study was to investigate whether social activities are associated with the development of cognitive impairment in Korean older people. Methods We analyzed data from the Korean National Longitudinal Study on Aging. A total of 2495 Korean community‐dwelling older adults (1163 men and 1332 women) aged between 65 and 79 years at the first wave of the Korean National Longitudinal Study on Aging were used for analysis. Cognitive function was assessed by the Mini‐Mental State Examination in 2006 and 2012. Multiple logistic regression analysis was carried out by adjusting covariates, such as age, sex, education, employment, Center for Epidemiological Studies‐Depression and instrumental activities of daily living scores, and weight loss. Results Among the participants, 951 participants (38.1%) showed cognitive decline. Compared with those who participated in a large number of group social activities, multivariate‐adjusted odds ratios of cognitive decline in those who participated in a moderate and small number of group activities were 1.18 (95% CI 0.93–1.48) and 1.80 (95% CI 1.16–1.90), respectively. Among six types of group social activities, two types (social club/café and alumni) showed a significant correlation with less cognitive decline; adjusted odds ratios of the group with a small number of activities were 1.31 (95% CI 1.09–1.56) and 1.46 (95% CI 1.10–1.93), respectively, compared with the group with a large number of activities. Personal social activities and the other four types of activities (religious, political, leisure and volunteer) did not affect the outcome. Conclusions Two social group activities (social club/café and alumni) were significantly associated with less cognitive decline in older Koreans. Geriatr Gerontol Int 2017; 17: 1191–1196.