BackgroundOlder individuals increasingly require emergency abdominal surgeries. They are susceptible to surgical stress and loss of independence in performing daily activities. We hypothesized that the psoas muscle volume would be significantly associated with postoperative functional decline (FD) in older patients undergoing emergency abdominal surgery and aimed to evaluate the use of the psoas muscle volume on computed tomography (CT) scans.MethodsA retrospective, single-center study of patients aged ≥ 65 years who had undergone emergency abdominal surgery between January 2019 and June 2021 was performed. We assessed patients’ activities of daily living using the Barthel Index. FD was defined as a ≥ 5-point decrease between preoperative and 28-day postoperative values. The psoas muscle volume was measured by CT, which was used for diagnosis, and normalized by height to calculate total psoas muscle index (TPI). We evaluated associations between FD and TPI using receiver operating characteristics (ROC) analysis and multiple logistic regression analysis.ResultsOf 238 eligible patients, 71 (29.8%) had clinical postoperative FD. Compared to the non-FD group, the FD group was significantly older and had a higher proportion of females, higher Charlson Comorbidity Index, lower body mass index, higher American Society of Anesthesiology score, lower serum albumin level, and lower TPI. ROC analyses revealed that TPI had the highest area under the curve (0.802; 95% confidence interval [CI], 0.75–0.86). A multivariable logistic regression model revealed that low TPI was an independent predictor of postoperative FD (odds ratio, 0.14; 95% CI, 0.06–0.32).ConclusionsTPI can predict postoperative FD due to emergency abdominal surgery. Identification of patients who are at high risk of FD before surgery may be useful for enhancing the regionalized system of care for emergency general surgery.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12893-023-02085-5.