The Korean government recently announced a plan to expand the mandatory disclosure of ESG information for all KOSPI-listed companies by 2030. While large corporations with resources can effectively comply with ESG-related regulations, SMEs (small and medium-sized enterprises) may become financially stressed by the rapid policy changes. Using the Korea Corporate Governance Service’s ESG data, this study examines the relationships between ESG performance and three financial indicators: financial performance, short-term corporate value, and long-term corporate value, according to corporate size. It demonstrates that, first, SMEs have a statistically significant positive correlation between ESG performance and financial performance, while large companies do not. Second, companies of all sizes have a statistically significant difference between ESG performance and long-term corporate value. However, in terms of direction, small companies show a inverse relationship of which ESG-related regulators should be prudent. Third, large and small companies do not show a significant relationship between ESG performance and short-term value, while mid-sized companies show a statistically significant negative relationship. Overall, the relationship between ESG performance and financial indicators is most pronounced and evident in mid-sized companies. In mid-sized companies, companies with good ESG performance have better financial performance and better long-term corporate value but show a negative relationship to short-term corporate value. The relationship for large and small companies is