Background: Hyponatremia is an unrecognized risk factor for adverse outcomes after cardiac surgery. We sought to study the prevalence of preoperative hyponatremia and its impact on short-term and long-term outcomes after cardiac surgery. Methods: Patients who had coronary artery bypass graft, valve, or coronary artery bypass graft and valve procedures from 2000 to 2016 and available preoperative serum sodium values within 30 days of the index procedure were included in the study. The effect of preoperative sodium on short-term and long-term outcomes was analyzed as a continuous and binary (hyponatremia [Na+ <135 mEq/L] versus no hyponatremia) predictor variable in multivariable regression models. Results: Preoperative hyponatremia was present in 9.9% of 16,238 patients with available sodium levels. Comorbidities were more common in patients with hyponatremia. Hyponatremia was independently associated with operative mortality (odds ratio [OR] = 1.80; 95% confidence interval [CI], 1.38-2.34; P < .001), long-term mortality (hazard ratio = 1.31; 95% CI, 1.21-1.40; P < .001), longer postoperative length of stay (hazard ratio = 1.35; 95% CI, 1.28-1.43; P < .001), renal failure (OR = 1.52; 95% CI, 1.20-1.93; P < .001), prolonged ventilation use (OR = 1.52; 95% CI, 1.30-1.78; P < .001), and stroke or transient ischemic attack (OR = 1.48; 95% CI, 1.09-2.02; P = .013). Severity of hyponatremia, as measured by sodium level, was similarly associated with increased risk for death and postoperative complications. Conclusions: Preoperative hyponatremia is relatively common and is associated with adverse short-term and long-term outcomes after cardiac surgery. Preoperative hyponatremia can be used independently from standard risk factors to identify high-risk patients for cardiac surgery.