Background Surgical wound classification has been the foundation for infectious risk assessment, perioperative protocol development, and surgical decision-making. The wound classification system categorizes all surgeries into: clean, clean/contaminated, contaminated, and dirty, with estimated postoperative rates of surgical site infection (SSI) being 1%–5%, 3%–11%, 10%–17%, and over 27%, respectively. The present study evaluates the associated rates of the SSI by wound classification using a large risk adjusted surgical patient database. Methods A cross-sectional study was performed using the American College of Surgeons National Surgical Quality Improvement Program (ACS-NSQIP) dataset between 2005 and 2008. All surgical cases that specified a wound class were included in our analysis. Patient demographics, hospital length of stay, preoperative risk factors, co-morbidities, and complication rates were compared across the different wound class categories. Surgical site infection rates for superficial, deep incisional, and organ/space infections were analyzed among the four wound classifications using multivariate logistic regression. Results A total of 634,426 cases were analyzed. From this sample, 49.7% were classified as clean, 35.0% clean/contaminated, 8.56% contaminated, and 6.7% dirty. When stratifying by wound classification, the clean, clean/contaminated, contaminated, and dirty wound classifications had superficial SSI rates of 1.76%, 3.94%, 4.75%, and 5.16%, respectively. The rates of deep incisional infections were 0.54%, 0.86%, 1.31%, and 2.1%. The rates for organ/space infection were 0.28%, 1.87%, 2.55%, and 4.54%. Conclusion Using ACS-NSQIP data, the present study demonstrates substantially lower rates of surgical site infections in the contaminated and dirty wound classifications than previously reported in the literature.