Background: Standard procedures to reduce the surgical site infection (SSI) rate after colorectal surgery have not been established. A prospective cohort study with 2 parallel study groups was performed to clarify the SSI rate after open surgery with and without a wound-edge protector (WEP) for colorectal disease. Methods: A total of 102 patients who underwent elective open surgery for colorectal disease between October 2012 and August 2014 were randomly assigned to a WEP group and a WEP (–) group. The primary endpoint was the SSI rate in both groups. Results: Fifty-one patients were assigned to the WEP group and 51 to the WEP (–) group. Two patients were excluded from this study. The rate of SSI with and without a WEP was 16 and 36% respectively (p = 0.021). Older age (p = 0.0073) and no WEP (p = 0.021) were risk factors for SSI after open surgery for colorectal diseases on univariate analysis. On multivariate analysis, both older age (p = 0.016) and no WEP (p = 0.012) were independent risk factors for SSI. Conclusion: The present randomized, phase II trial found an SSI rate of 16% with use of a WEP. The SSI rate was lower in the WEP group than in the WEP (–) group. A WEP may reduce the SSI rate after open surgery for colorectal disease in Japanese patients.