BackgroundSinonasal malignant tumors (SNMTs) have a high recurrence risk, which is responsible for the poor prognosis of patients. Assessing recurrence risk in SNMT patients is a current problem.PurposeTo establish an MRI‐based radiomics nomogram for assessing relapse risk in patients with SNMT.Study TypeRetrospective.PopulationA total of 143 patients with 68.5% females (development/validation set, 98/45 patients).Field Strength/SequenceA 1.5‐T and 3‐T, fat‐suppressed fast spin echo (FSE) T2‐weighted imaging (FS‐T2WI), FSE T1‐weighted imaging (T1WI), and FSE contrast‐enhanced T1WI (T1WI + C).AssessmentThree MRI sequences were used to manually delineate the region of interest. Three radiomics signatures (T1WI and FS‐T2WI sequences, T1WI + C sequence, and three sequences combined) were built through dimensional reduction of high‐dimensional features. The clinical model was built based on clinical and MRI features. The Ki‐67‐based and tumor‐node‐metastasis (TNM) model were established for comparison. The radiomics nomogram was built by combining the clinical model and best radiomics signature. The relapse‐free survival analysis was used among 143 patients.Statistical TestsThe intraclass/interclass correlation coefficients, univariate/multivariate Cox regression analysis, least absolute shrinkage and selection operator Cox regression algorithm, concordance index (C index), area under the curve (AUC), integrated Brier score (IBS), DeLong test, Kaplan–Meier curve, log‐rank test, optimal cutoff values. A P value < 0.05 was considered statistically significant.ResultsThe T1 + C‐based radiomics signature had best prognostic ability than the other two signatures (T1WI and FS‐T2WI sequences, and three sequences combined). The radiomics nomogram had better prognostic ability and less error than the clinical model, Ki‐67‐based model, and TNM model (C index, 0.732; AUC, 0.765; IBS, 0.185 in the validation set). The cutoff values were 0.2 and 0.7 and then the cumulative risk rates were calculated.Data ConclusionA radiomics nomogram for assessing relapse risk in patients with SNMT may provide better prognostic ability than the clinical model, Ki‐67‐based model, and TNM model.Evidence Level3.Technical EfficacyStage 5.