This study examines the effect of retirement on the cognitive ability of aging Koreans.Using six (2006–2016) waves of the Korean Longitudinal Study of Aging (KLoSA), we find the significant negative effect of retirement on cognitive ability. After retirement,individuals experience a decline in their cognitive ability scores by approximately 1% when compared with the average score. We also investigate the heterogeneity across age, gender,education, and job characteristics. The negative influence of retirement is pronounced and significant for those who are aged 68–80, are females, have lowers levels of education, are non-wage-and-salary workers, and are blue-collar workers.