Objective Our objective was to investigate the association between anastomotic leakage (AL) and hyponatremia after colorectal cancer surgery. Methods All anastomoses in colorectal cancer surgery performed in our hospital between January 2015 and December 2017 were retrospectively identified. According to the diagnostic criteria of AL, the patients were divided into an AL group and a no anastomotic leakage (NAL) group. Results We reviewed records of 498 consecutive colorectal cancer patients. The total incidence of AL was 5.4%. Postoperative serum sodium levels differed significantly: 137.63 +/- 4.29 and 139.81 +/- 3.41 mmol/L in the AL and NAL groups, respectively. By using area under the receiver-operating characteristic (auROC) curves, we determined the optimum postoperative serum sodium cut-off to be 139.5 mmol/L and redefined hyponatremia as postoperative serum sodium <139.5 mmol/L. Redefined hyponatremia had an auROC of 0.65, corresponding to a 97.2% negative predictive value. The negative predictive value reached 99.1% when serum sodium level was combined with leukocytosis. Multivariable analysis found that redefined hyponatremia (odds ratio, 1.176) was an independent predictive factor for AL. Conclusions Redefined hyponatremia has good negative predictive value for AL diagnosis after colorectal cancer surgery and could be used as a marker to exclude the diagnosis.