<h3>Objectives</h3> Malnutrition is associated with adverse surgical outcomes. The Geriatric Nutritional Risk Index, a nutritional screening tool, could be a predictor of postoperative atrial fibrillation and outcomes in cardiac surgery. <h3>Design</h3> The authors enrolled 292 patients who underwent cardiac surgery and showed postoperative atrial fibrillation during hospitalization, and assessed mortality and readmission during a 90-day follow-up period. <h3>Setting</h3> A large academic hospital in China. <h3>Participants</h3> Participants were divided into low-risk (Geriatric Nutritional Risk Index <98) and no-risk malnutrition groups (Geriatric Nutritional Risk Index ≥98). <h3>Interventions</h3> Univariate and multivariate logistic regression analyses were used to validate the role of the Geriatric Nutritional Risk Index in predicting postoperative atrial fibrillation. Kaplan–Meier analysis was used to examine the effect of Geriatric Nutritional Risk Index scores on mortality and readmission after cardiac surgery. <h3>Measurements and Main Results</h3> A total of 136 patients were in the low-risk malnutrition group. Postoperative atrial fibrillation was more common in the low-risk group (63.2% <i>v</i> 28.8%, p < 0.001). Duration of intensive care unit stays and hospitalization were significantly longer than in the no-risk group (44 [43] <i>v</i> 39 [47] hours, p < 0.001; 18 [7] <i>v</i> 15 [6] days, p < 0.001). Multivariate analysis showed that Geriatric Nutritional Risk Index, age, and NYHA class independently predicted postoperative atrial fibrillation. Kaplan–Meier analysis showed that 90-day all-cause readmission, but not 90-day all-cause mortality, was different between groups based on the Geriatric Nutritional Risk Index (log-rank p < 0.001, log-rank p = 0.09). <h3>Conclusions</h3> The easily accessible Geriatric Nutritional Risk Index used for nutritional assessment before cardiac surgery has a predictive role in postoperative atrial fibrillation, and is related to short-term readmission.