Background: The aim of this study is to establish a model to predict the overall survival (OS) and stratify the risk of postoperative patients with undifferentiated sarcoma.Methods: A total of 452 postoperative patients with undifferentiated sarcoma in the trunk and extremity from the Surveillance, Epidemiology, and End Results database were enrolled as the training cohort. We collected a group of 163 undifferentiated sarcoma patients from our center as the external validation cohort. Cox proportional hazards regression model was used to screen survival-associated factors for the construction of the nomogram. Concordance-indexes (C-indexes), calibration curves, and receiver operating characteristics (ROCs) curves were applied for the discrimination and calibration of the nomogram. The cutoff value of nomogram-based total points was applied to stratify the risk of patients.Results: A nomogram was developed incorporating four independent factors: age, tumor site, eighth AJCC stage, and radiotherapy. The nomogram showed good prognostic accuracy and excellent agreement in the training and validation cohort, with C-indexes of .701 (95% confidence interval [CI]: .683-.719) and .700 (95% CI: 0.659-.741), respectively. Furthermore, we identified the best cutoff value of nomogram total points (103.2) as the predicted risk and divided the patients into a high-risk group and a low-risk group. Significant differences in OS between the two groups were indicated in the training cohort and external validation cohort, showing the appreciable clinical validity and clinical utility of the nomogram (P < .001).Conclusion: This nomogram provides an insightful and applicable tool for individual evaluations and the distinguishment of risk for patients with undifferentiated sarcoma.