Abstract: Objective: This systematic review aimed to evaluate the efficacy and safety of preoperative nutrition-based interventions on pre-, intra-, and postoperative outcomes in children undergoing cardiac surgical procedures. Methods: CENTRAL, MEDLINE, and EMBASE were systematically searched for interventional and observational studies comparing any nutritional preoperative intervention with a control or alternative strategy in pediatric patients undergoing cardiac surgery, up to July 2025. The main outcome was postoperative length of stay in the intensive care unit (ICU). The certainty of evidence was assessed using the GRADE approach. Results: Nineteen studies were included (8 randomized controlled trials [RCTs], 1 non-randomized trial, and 10 observational studies), evaluating heterogeneous interventions or exposures, including fatty acids, vitamin D supplementation, and structured preoperative nutritional protocols. Two RCTs demonstrated shorter ICU and hospital stays with extended preoperative nutritional support (2 weeks vs 1 week; n = 40; and 1 month vs no support; n = 80). Observational data indicated an association between preoperative nutritional support and reduced hospital length of stay (meta-analysis of 4 studies; n = 278) as well as fewer days to achieve full enteral feeding postoperatively (meta-analysis of 3 studies; n = 138). No significant difference in postoperative ICU stay was observed between groups (meta-analysis of 2 studies; n = 175). No intervention-related serious adverse events were reported. The overall certainty of evidence was very low. Conclusions: This systematic review provides very low–certainty evidence suggesting that preoperative nutrition-based interventions in children undergoing cardiac surgery are safe and may offer clinical benefits. Substantial heterogeneity across studies underscores the need for well-designed trials and standardized preoperative nutritional protocols.