Background <p>Gut microbiota and intestinal environment are crucial in minimizing the risk for postoperative infectious complications of major gastrointestinal cancer surgery. However, comprehensive studies across multiple cancer types with large samples are limited. This study aimed to identify preoperative factors associated with the development of infectious complications after such surgeries.</p> Methods <p>This large-scale exploratory study examined 381 patients who underwent surgery for seven types of gastrointestinal cancer. Preoperative fecal samples were analyzed for fecal organic acid concentrations using high-performance liquid chromatography and gut microbiota composition 16S rRNA gene amplicon-sequencing. The relationships between specific gut bacteria, lactic acid concentrations, and postoperative infectious complications were analyzed.</p> Results <p>Fecal lactic acid concentrations were significantly higher in patients who experienced postoperative infectious complications, indicating that lactic acid concentration is an independent risk factor. Greater preoperative abundances of <i>Akkermansia muciniphila</i> and <i>Lactococcus lactis</i> were associated with a lower incidence of postoperative infectious complications. Additionally, <i>A. muciniphila</i> abundance was positively correlated with preoperative blood neutrophil and lymphocyte counts.</p> Conclusions <p>Fecal lactic acid concentrations, <i>A. muciniphila</i>, and <i>L. lactis </i>were identified as factors associated with the development or suppression of postoperative infectious complications. These findings suggest novel avenues for predicting and managing infection risks through preoperative assessments and microbiota modulation.</p>