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Background: Preoperative hyponatremia is prevalent in patients undergoing surgical procedures, but it is uncertain if hyponatremia will lead to increased risk of surgical mortality and morbidity. Methods: A systematic search of Medline (PubMed), Embase, and Cochrane Library from inception through July 2, 2021, was performed. Fulllength articles that reported on the association between surgical outcomes among adults aged ≥18 years with documented preoperative hyponatremia were included. Findings: We identified 32 observational studies comprising 1 301 346 participants. All studies had low risk of bias. When adjusted for covariates, patients with hyponatremia had significantly higher odds of developing major complications (defined as a composite measure of 9 major complications) compared with patients with normal sodium concentrations (adjusted odds ratio=12711.37; 95% CI, 1.23-1.53; I²=78%; N =10). Additionally, patients with preoperative hyponatremia also significantly higher hazards of early mortality (<90 days) compared with patients with normonatremia (adjusted hazard ratio=1.27; 95% CI, 1.13-1.43; I²=97%; N =10) after adjustment for covariates. Preoperative hyponatremia also had significant associations with respiratory, renal, and septic complications. In terms of prognostic performance, preoperative hyponatremia performed adequately in predicting major complications in surgical patients (area under the curve=0.70; negative likelihood ratio, 0.90) with a specificity of 88% and a sensitivity of 25%. Interpretation: Our meta-analysis suggests that preoperative hyponatremia is associated with poorer early mortality and major morbidity outcomes in surgical patients. Hyponatremia is also a specific prognosticator for major complications in surgical patients, reiterating its potential use as a clinical indicator of poor outcomes. [ABSTRACT FROM AUTHOR]