Background: Bacterial infection following gastrointestinal surgery remains a common morbidity. The aim of this study was to estimate the effect of the perioperative use of probiotics and synbiotics on postoperative infections. Materials and Methods: We searched PubMed, Embase, and the Cochrane Library to identify pertinent randomized controlled trials (RCTs). The primary outcome was postoperative infection rate. The secondary outcomes were length of hospital and intensive care unit (ICU) stay, length of antibiotic therapy, and mortality. The pooled outcomes were calculated using random effects models. Results: Twenty-eight RCTs involving 2511 patients were included in this systematic review. The incidence of infectious complications was lower among patients who received probiotics/synbiotics than among the controls (odds ratio [OR] = 0.35; 95% confidence interval [CI], 0.24–0.50), particularly regarding respiratory (OR = 0.44; 95% CI, 0.28–0.68), urinary tract (OR = 0.30; 95% CI, 0.16–0.55), and wound infections (OR = 0.58; 95% CI, 0.42–0.80). The lengths of hospital stay (mean difference [MD] = −3.20; 95% CI, −4.87 to −1.54) and duration of antibiotic therapy (MD = −3.40; 95% CI, −4.67 to −2.13) were shorter for patients who received probiotics/synbiotics than for controls. There were no significant differences in mortality (OR = 1.19; 95% CI, 0.52–2.74) or length of ICU stay (MD = −0.46; 95% CI, −1.07 to 0.14) between the compared groups. Conclusion: Probiotics and synbiotics may prevent postoperative infections in patients undergoing gastrointestinal surgery. However, the results need to be interpreted with caution due to the risk of bias in the included studies and the potential publication bias.