Purpose This paper aims to analyze the market efficiency of socially responsible investment in Korea. The authors used the daily price of the Dow Jones Sustainability Index Korea between January 2006 and December 2015. Design/methodology/approach To analyze the unpredictability of the returns, the authors conducted runs tests, such as the Dickey–Fuller test, the Philip–Perron test, the variance ratio test and autocorrelation tests. These tests investigate whether the future price of socially responsible investment in Korea is dependent on its previous price. If the relationship is dependent, this will violate the theory of weak form of efficient market hypothesis which explains that the past price movements and data do not affect stock prices. Therefore, investors cannot gain any abnormal return by extrapolating the historical data. Findings The results suggest that the weak form of the efficient market hypothesis is not valid for the Dow Jones Sustainability Index Korea. This implies that the future price of the index is correlated with past prices. Hence, the future movement of socially responsible investment in Korea can be predicted and enables socially responsible investors to gain abnormal returns. Originality/value This is the first study to investigate the market efficiency of socially responsible investment in Korea.