Abstract Background Infectious complications occur in 4–22 per cent of patients undergoing surgical resection of malignant solid tumours. Improving the patient's immune system in relation to oncological surgery with immunonutrition may play an important role in reducing postoperative infections. A meta-analysis was undertaken to evaluate the potential clinical benefits of immunonutrition on postoperative infections and 30-day mortality in patients undergoing oncological surgery. Methods PubMed, Embase and Cochrane Library databases were searched to identify eligible studies. Eligible studies had to include patients undergoing elective curative surgery for a solid malignant tumour and receiving immunonutrition orally before surgery, including patients who continued immunonutrition into the postoperative period. The main outcome was overall infectious complications; secondary outcomes were surgical-site infection (SSI) and 30-day mortality, described by relative risk (RR) with trial sequential analysis (TSA). Risk of bias was assessed according to Cochrane methodology. Results Some 22 RCTs with 2159 participants were eligible for meta-analysis. Compared with the control group, immunonutrition reduced overall infectious complications (RR 0·58, 95 per cent c.i. 0·48 to 0·70; I2 = 7 per cent; TSA-adjusted 95 per cent c.i. 0·28 to 1·21) and SSI (RR 0·65, 95 per cent c.i. 0·50 to 0·85; I2 = 0 per cent; TSA-adjusted 95 per cent c.i. 0·21 to 2·04). Thirty-day mortality was not altered by immunonutrition (RR 0·69, 0·33 to 1·40; I2 = 0 per cent). Conclusion Immunonutrition reduced overall infectious complications, even after controlling for random error, and also reduced SSI. The quality of evidence was moderate, and mortality was not affected by immunonutrition (low quality). Oral immunonutrition merits consideration as a means of reducing overall infectious complications after cancer surgery.