Objective To explore the impact of mandatory adoption of diagnosis‐related groups (DRGs) on the use of outpatient care in Korea. Data Sources National Health Claim data from 2,022 hospitals and 1,029,101 admission cases during 2011–2014: tonsillectomy/adenoidectomy, inguinal/femoral hernia operation, and hemorrhoidectomy. Study Design Outcome variables included probability of outpatient visit, number of outpatient visits, and outpatient medical expenditures within 30 days. Presurgery examination before hospitalization for surgery, including basic and other examination, was conducted to evaluate a possible shift in health care service. A difference‐in‐difference research design was used to evaluate the impact of the DRG system on the use of outpatient care. Principal Findings Before the introduction of the DRG system, 384,609 (91.1 percent) participants used an outpatient clinic either before or after hospitalization. In our study, the number of outpatient visits and outpatient medical expenditures within 30 days increased after mandatory adoption of the DRG system. After adoption of the DRG system, volume and costs for presurgery examinations increased before hospitalization. Conclusion We observed a spillover effect after mandatory adoption of the DRG system. A future payment system should be designed for spillover effects, and the introduction of a new payment system that expands the DRG‐based reimbursement system should be considered.