The purpose of this paper is to examine the association between corporate environmental responsibility (CER) activities and investment efficiency as measured by overinvestment, and whether the industry-level competition affects this association. We investigate a sample of 2285 non-financial firms with fiscal year-end in December listed in the Korea Stock Exchange Market for the period of 2013–2018, measuring the investment efficiency by overinvestment model. Using environmental scores from the Korea Corporate Governance Service to measure CER activities, we show that, on average, firms can decrease overinvestment behavior through CER activities in South Korea. Moreover, in firms in a highly competitive market, the negative association between CER activities and overinvestment is pronounced, indicating that strong product market competition are effective in monitoring managerial opportunistic behavior. These results are robust, even after controlling for different setting and alternative CER. These findings also suggest that the relationship between CER and overinvestment appears to be benefit firms that are sound and sustainable and honestly present their financial information.