Primary malignant non-Hodgkin's lymphoma of the muscle is rare. Currently, imaging tools are necessary to enable its diagnosis. Herein, we report the case of a patient who presented with swelling and pain in the right thigh and pelvis. Computed tomography findings revealed isodense masses in the patient's right thigh and left iliacus muscle, leading to the initial diagnosis of either primary muscular lymphoma or soft tissue sarcoma. Further investigation with magnetic resonance imaging was done, and a biopsy was performed. The ensuing histological diagnosis was that of diffuse large B-cell lymphoma.