Cardiac surgery, particularly procedures involving cardiopulmonary bypass (CPB), is associated with a high risk of postoperative complications, including systemic inflammatory response syndrome (SIRS), postoperative atrial fibrillation (POAF), and infection. Growing evidence suggests that the gut–heart axis, through mechanisms involving intestinal barrier integrity and gut microbiota homeostasis, may influence these outcomes. This review summarizes the relationship between gut microbiota composition and the inflammatory response in patients undergoing cardiac surgery and the extent to which these alterations impact clinical outcomes. The reviewed studies consistently show that cardiac surgery induces notable alterations in microbial diversity and composition during the perioperative period. These changes, indicative of dysbiosis, are characterized by a reduction in health-associated bacteria such as Blautia, Faecalibacterium, and Bifidobacterium and an increase in opportunistic pathogens. Inflammatory biomarkers were frequently elevated postoperatively, even in patients without evident complications. Key microbial metabolites and biomarkers, including short-chain fatty acids (SCFAs), trimethylamine N-oxide (TMAO), and bile acids (BAs), were implicated in modulating inflammation and clinical outcomes. Additionally, vitamin D deficiency emerged as a contributing factor, correlating with increased systemic inflammation and a higher incidence of POAF. The findings suggest that gut microbiota composition prior to surgery may influence the severity of the postoperative inflammatory response and that perioperative modulation of the gut microbiota could represent a novel approach to improving surgical outcomes. However, the relationship between dysbiosis and acute illness in surgical patients is confounded by factors such as antibiotic use and other perioperative interventions. Large-scale, standardized clinical studies are needed to better define these interactions and guide future therapeutic strategies in cardiac surgery.