Background Esophagectomy is an invasive and complication-prone surgical procedure. Therefore, a tool that can predict the occurrence of postoperative complications may be useful for perioperative management. In this study, we investigated whether the modified frailty index (mFI) could be a useful tool for predicting the postoperative complications of esophagectomy. Methods In this study, 162 patients who underwent curative esophagectomy for esophageal squamous cell carcinoma from 2004 to 2019 at our institution were included. The patients were divided into the high mFI (≥ 0.27) and low mFI (< 0.27) groups, and the short-term postoperative outcomes of each group were examined retrospectively.Results Regarding background factors, age and the American Society of Anesthesiologists physical status classification were significantly higher in the high mFI group (P = 0.049 and P = 0.002, respectively); however, the other items were not significantly different between the two groups. Regarding surgical outcomes, no significant differences in operative time, blood loss, and hospital stay were observed between the two groups. Regarding postoperative complications, pneumonia was significantly more common in the high mFI group (P = 0.035). In multivariate analysis, high mFI (P = 0.034) was an independent predictor of pneumonia, along with operative time ≥ 613 min (P = 0.03) and preoperative BMI < 20.48 (P = 0.006). Conclusion The mFI is useful for predicting pneumonia after esophagectomy.