We examine institutional blockholders’ active monitoring influence using a proprietary corporate governance score (CGS) provided by the Korea Corporate Governance Service (KCGS). We find that institutional blockholders effectively exert monitoring influence to improve CSG scores of investee firms. The evidence of effective monitoring is particularly evident for domestic institutional blockholders and is strongest in the shareholder rights category of the CSG score. Consistent with domestic blockholders having an informational advantage over their foreign counterparts, the evidence of active monitoring is stronger (weaker) in firms with lower (higher) earnings management (higher information quality) and for firms with lower (higher) stock liquidity. Our robust findings shed light on the specific monitoring role of institutional blockholders in emerging markets, where sound corporate governance is essential to firms’ long-term sustainability.