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Introduction: Erythema nodosum (EN) is the most common form of panniculitis. EN can be idiopathic or secondary to an underlying systemic disease, infection, drug use, or tumor. CD5-positive diffuse large B-cell lymphoma (CD5+ DLBCL) is a relapsed and refractory lymphoma, and further understanding of its pathology is required. We report a case of newly diagnosed CD5+ DLBCL with concomitant EN. Within the scope of our search, there were no reports of CD5+ DLBCL complicated with EN. Case Presentation: A 79-year-old woman experienced swelling, warmth, redness, and pain in both legs and a mass lesion on the right side of the back at almost the same time. The respective lesions were diagnosed as EN and CD5+ DLBCL by biopsy. With chemotherapy, the lymphoma and EN improved in parallel courses. The patient has completed scheduled chemotherapy, and there has been no recurrence of swelling in the legs or mass on the right side of the back. Discussion: The lymphoma and EN developed simultaneously and followed a parallel clinical course after chemotherapy, suggesting that EN was a paraneoplastic symptom of CD5+ DLBCL. Recognizing and treating underlying malignancies in patients presenting with EN is crucial. [ABSTRACT FROM AUTHOR]