Most pelvic tumors originate from the organs. Less commonly, tumors can arise from the various anatomic pelvic compartments and are comprised of mesenchymal tissue: muscles, connective tissue, vessels, lymphatics, and fat. Among some of the rarer entities are benign tumors (eg, angiomyxoma, cellular angiofibroma, and desmoid fibromatosis), malignant tumors (eg, sarcoma), and tumors that can manifest as benign or malignant (eg, solitary fibrous tumor or nerve sheath tumor). Because these tumors are uncommon and often manifest with nonspecific clinical features, imaging (usually MRI) is an initial step in the evaluation. Radiologists interpreting these images are asked to help narrow the differential diagnosis and assess the likelihood of malignancy for treatment planning. Thus, the MRI report should include the imaging features that would indicate the underlying tissue histology for pathologic diagnosis as well as a description of the anatomic extent and pattern of growth. The authors describe multiple locally aggressive benign and malignant mesenchymal tumors and highlight characteristic clinical and imaging features that enable the radiologist to narrow the differential diagnosis. The anatomic spaces of the pelvis are reviewed with illustrations to aid the radiologist in describing these tumors, which often span multiple pelvic compartments. Tumor appearance at T2-weighted, diffusion-weighted, and postcontrast MRI is summarized and illustrated with correlation at CT or fluorodeoxyglucose PET/CT, when available. MRI features that correspond to specific types of tissue (eg, myxoid, fibrous, or vascular) are highlighted and correlated with images from pathologic evaluation. Online supplemental material is available for this article. ©RSNA, 2021.