Purpose Although surgical site infections (SSI) are a major postoperative complication of appendectomy, few studies have focused on the risk factors for SSI. In this study, we investigated the risk factors for SSI in patients who had undergone appendectomy. Methods Three hundred patients who had undergone open appendectomy were enrolled. The patients were divided into two groups based on the presence or absence of SSI. A statistical analysis was performed to assess the clinical features associated with SSI after appendectomy. Results A multivariate analysis using the results of univariate analyses revealed that the serum C-reactive protein (CRP) level (≤65/>65, mg/l), length of the operation (≤80/>80, min) and pathology (catarrhal, phlegmonous/gangrenous) were associated with SSI. Among these three clinical features, only the CRP level was found to predict the risk of SSI prior to appendectomy (odds ratio 3.797; 95 % confidence intervals 1.305–11.04; P  = 0.014). Conclusion Preoperative elevation of the serum CRP level (>65 mg/l) is a valuable predictor of SSI in patients undergoing appendectomy.