Background: Preoperative prediction of the soft tissue sarcomas (STSs) grade is important for treatment decisions. To preoperatively distinguish low-grade (grades I and II) and high-grade (grade III) STSs, we developed and validated the performance of a magnetic resonance imaging (MRI)-based radiomics nomogram.Methods: Patients with an STS based on the French Federation of Cancer Centers Sarcoma Group grading system at two independent institutions were enrolled (training set, n = 109; external validation set, n = 71). The minimum redundancy maximum relevance method and least absolute shrinkage and selection operator logistic regression were used to process feature selection and radiomics signature development. Three radiomics signature models were constructed based on T1-weighted imaging (RS-T1 model) and fat-suppressed T2-weighted imaging sequences (RS-FST2 model) and their combination (RS-Combined model). Model performance (discrimination capability, calibration curve, and clinical usefulness) was evaluated in the external validation set. Results: The RS-T1 model, RS-FST2 model, and RS-Combined model achieved predictive abilities with area under the receiver operating characteristic curves (AUCs) of 0.645, 0.641, and 0.829, respectively, in the external validation set. The nomogram, incorporating significant clinical factors and the RS-Combined model, showed extremely high predictive ability in the training set and external validation set with AUCs of 0.916 (95% confidence interval, 0.866–0.966) and 0.879 (0.791–0.967), respectively. The nomogram achieved significant patient stratification.Conclusions: The proposed noninvasive MRI-based radiomics nomogram shows superior predictive performance in differentiating low-grade from high-grade STS.