Purpose: This study examined the mediating effect of bowel function symptoms in the relationship between social support and uncertainty in rectal cancer patients. Methods: In total, 132 rectal cancer patients were recruited online. Data were collected from October 1, 2020 to October 31, 2020 using on-line questionnaires and were analyzed using the t-test, analysis of variance, Pearson correlation coefficients and bootstrapping for mediation. Results: Social support had a significant and positive effect on bowel function symptoms and a negative effect on uncertainty. Bowel function symptoms had a significant negative effect on uncertainty; and showed a statistically significant mediating effect in the relationship between social support and uncertainty. In other words, social support had direct and indirect effects on uncertainty. Conclusion: In rectal cancer patients, social support attenuated bowel function symptoms and showed direct and indirect effects on uncertainty. Therefore, to increase the social support of rectal cancer patients, the patients’ family, friends, and meaningful others should be involved in providing nursing. In order to reduce bowel function symptoms, which have a mediating effect, rectal cancer patients need to be well aware of the symptoms of bowel function that change after surgery. Systematic pre- and post- operative education would be help reduce uncertainty.