objective. To evaluate the risk factors for surgical site infection (SSI) after gastric surgery in patients in Korea. design. A nationwide prospective multicenter study. setting. Twenty university-affiliated hospitals in Korea. methods. The Korean Nosocomial Infections Surveillance System (KONIS), a Web-based system, was developed. Patients in 20 Korean hospitals from 2007 to 2009 were prospectively monitored for SSI for up to 30 days after gastric surgery. Demographic data, hospital characteristics, and potential perioperative risk factors were collected and analyzed, using multivariate logistic regression models. results. Of the 4,238 case patients monitored, 64.9% (2,752) were male, and mean age (±SD) was 58.8 (±12.3) years. The SSI rates were 2.92, 6.45, and 10.87 per 100 operations for the National Nosocomial Infections Surveillance system risk index categories of 0, 1, and 2 or 3, respectively. The majority (69.4%) of the SSIs observed were organ or space SSIs. The most frequently isolated microorganisms were Staphylococcus aureus and Klebsiella pneumoniae. Male sex (odds ratio [OR], 1.67 [95% confidence interval (CI), 1.09-2.58]), increased operation time (1.20 [1.07-1.34] per 1-hour increase), reoperation (7.27 [3.68-14.38]), combined multiple procedures (1.79 [1.13-2.83]), prophylactic administration of the first antibiotic dose after skin incision (3.00 [1.09-8.23]), and prolonged duration (≥7 days) of surgical antibiotic prophylaxis (SAP; 2.70 [1.26-5.64]) were independently associated with increased risk of SSI. conclusions. Male sex, inappropriate SAP, and operation-related variables are independent risk factors for SSI after gastric surgery.