Background: Malignant lymphoma is the most common hematological malignancy; however, primary central nervous system lymphoma accounts for only a small percentage of non-Hodgkin lymphoma (NHL). Among these, primary cauda equina lymphoma (CEL) is extremely uncommon. Its rarity and atypical clinical presentation often make diagnosis challenging. Case Presentation: An 80-year-old man presented with progressive gait disturbance, lower-extremity weakness, and numbness. MRI revealed diffuse swelling and homogeneous gadolinium enhancement of the cauda equina at T12–L1; additionally, CSF cytology identified malignant lymphocytes. Open biopsy confirmed a diagnosis of diffuse large B-cell lymphoma. At diagnosis, the patient was classified as Ann Arbor stage IV, and the clinical parameters corresponded to a high-risk International Prognostic Index (IPI) category. The patient received five courses of immunochemotherapy with rituximab, methotrexate, vincristine, and procarbazine (R-MPV), resulting in marked radiological improvement and functional recovery, achieving a complete response. However, consolidation therapy was discontinued as the patient did not wish to continue. Unfortunately, intracranial relapse occurred four months later, and the patient ultimately succumbed to infectious complications. Only 29 cases of primary CEL have been reported. For all cases, a biopsy with histopathological examination is required for a definitive diagnosis. Currently, combined chemotherapy and radiotherapy are considered the standard treatment. This case was diagnosed through nerve biopsy with cauda equina at T12 to L1 levels, and immunochemotherapy successfully reduced the lesion while improving lower extremity function. Conclusions: Despite the considerable burden on patients, nerve biopsy is necessary for primary CEL to obtain a diagnosis and an early therapeutic approach for both neurological and vital prognoses.