Republic of Korea launched its national emissions trading system (K-ETS) in 2015 and is currently in Phase 3 of implementation. As one of the most prominent examples of a successfully established national ETS, the K-ETS covers 73.5% of the country’s total greenhouse gas (GHG) emissions during Phase 3, playing a pivotal role in achieving national mitigation goals. This study reviews the institutional development of the K-ETS over the past decade and assesses the performance and limitations of its key components—allocation, trading, and compliance—based on operational data. It also provides a quantitative assessment of emissions reporting and verification procedures, the use of Korean Credit Unit (KCU), and the application of flexibility mechanisms such as banking and borrowing. The analysis highlights several institutional improvements, including the expansion of auctioned allocations, adoption of the benchmarking (BM) method, and introduction of market facilitators. These reforms have contributed to a significant increase in market activity, driven by a broader range of participants, greater use of flexibility mechanisms, and influence of policy events. Based on the findings, the paper proposes policy directions to enhance market liquidity, institutional credibility, and mitigation effectiveness. The study offers valuable insights for designing the forthcoming Phase 4 of the K-ETS and supporting Republic of Korea’s carbon neutrality target for 2050.