Introduction: Recently, accelerometers have received much attention around the world. This study examined whether the preoperative physical activity level measured by an accelerometer could be a useful predictor of post-hepatectomy complications. Methods: Between December 2016 and December 2020, the physical activity levels of 185 patients were measured using an accelerometer 3 days before hepatectomy and from postoperative day 1 to 7. The patients without postoperative complications (n = 153) and those with postoperative complications (n = 32) were compared using either the χ<sup>2</sup> test or Fisher’s exact test for nominal variables; continuous variables were analyzed using either Student’s t test or Mann-Whitney U test. Differences were considered statistically significant when the p value was <0.05. Risk factors for postoperative complications following hepatectomy were also investigated. Results: The number of patients with an anatomical resection was significantly higher in patients with postoperative complications (p = 0.001). Furthermore, laparoscopic hepatectomy was performed in 65.4% of patients without postoperative complications and in 25.0% of those with postoperative complications; the difference was statistically significant (p < 0.001). The average preoperative physical activity level was 150.6 kcal/day in patients without postoperative complications and 84.5 kcal/day in those with postoperative complications (p = 0.001). Multivariate analysis identified blood loss, operative time, and preoperative physical activity level as independent risk factors for postoperative complications. Discussion/Conclusion: Patients with lower preoperative physical activity levels are at a high risk of developing postoperative complications after hepatectomy. Hence, preoperative physical activity level measurement may be useful in predicting post-hepatectomy complications.