Increasing evidences indicate that the controlling nutritional status (CONUT) score is associated with cancer prognosis. This study was conducted to investigate the prognostic significance of pretreatment CONUT score on patients with various malignant tumors. The correlation between CONUT score and clinical outcomes of tumor patients were studied by electronic literature retrieval. Pooled hazard ratios (HR), odds ratios (OR) and 95% confidence intervals (95% CI) were calculated to clarify the conclusion. Subgroup analysis were conducted in line with cancer type, cancer stage, treatment, sample size and cut-off value. A total of 62 studies involving 25224 patients were included in this study. Pooled analysis showed that higher CONUT scores were associated with shorter overall survival (HR 1.62, 95% CI 1.45-1.79, p < 0.001), cancer-specific survival (HR 1.80, 95% CI 1.48-2.13, p < 0.001), progress/recurrence-free survival (HR 1.54, 95% CI 1.23-1.85, p < 0.001) and disease-free survival (HR 1.56, 95% CI 1.39-1.74, p < 0.001). In addition, high CONUT score was correlated to higher incidence of postoperative complications (OR 1.64, 95% CI 1.38-1.95, p < 0.001) and mortality (OR 4.22, 95% CI 2.22-8.02, p < 0.001). Consequently, the pretreatment CONUT score is a valuable indicator to predict the clinical outcomes of patients with various malignant tumors.