Background/Aim: The Barthel index and Katz index are useful in assessing the physical function status of patients. We prospectively evaluated the relationship between the Barthel and Katz indices and postoperative complications in older patients undergoing gastrointestinal surgery. Patients and Methods: We enrolled 250 patients aged ≥65 years who underwent gastrointestinal surgery between September 2018 and April 2020. Postoperative complications within 30 days after surgery were categorized using the Clavien-Dindo classification. The primary endpoint was the incidence of postoperative complications of Clavien-Dindo grade ≥II. Results: Of the 250 patients (age, 74.1±6.2 years), 149 (59.6%) were male and 101 (40.4%) were female. The Barthel index was <100 in 49 patients (19.6%), and the Katz index was B-G in 23 patients (9.2%). Seventy-five patients (30%) developed postoperative complications of Clavien-Dindo grade ≥I, and 72 patients (28.8%) developed complications of Clavien-Dindo grade ≥II within 30 days after surgery. A Barthel index of <100 was more common in the group with complications, while the Katz index did not differ. A Barthel index of <100 was also a significant multivariate predictor of complications of Clavien-Dindo grade ≥II. Conclusion: The Barthel index is a useful predictor of postoperative complications in older individuals undergoing gastrointestinal surgery.