Introduction Primary ovarian non-Hodgkin's lymphoma is a very rare disease. Lymphoma presenting as an ovarian mass with initial manifestation is even rarer. We report a case of primary Non-Hodgkin's Lymphoma (NHL) of left ovary Matériel et Méthodes Case report Résultats A 38-year-old female presented with distension of abdomen and lower abdominal back pain. USG and CT-scan imaging suggested provisional diagnosis of ovarian tumour. She underwent left salpingo-oophorectomy, multiple biopsies from the right ovary, total omentectomy, pelvic and paraortic lymphadenectomy, appendectomy and curettage.The diagnosis of malignant lymphoma was made by histopathological examination of the excised tissue along with immunohistochemistry by using LCA, CD20, cytokeratin & CD3. Final histology showed diffuse large B-cell primary ovarian non-Hodgkin's lymphoma. Postoperatively, the patient underwent chemotherapy with the CHOP regimen in combination with rituximab. Five years after initial diagnosis, the patient remains well with normal menstrual cycle, without evidence of recurrence Conclusion Although uncommon, lymphoma should be included in the differential diagnosis of gynecologic malignancies because of a favorable outcome when properly diagnosed and treated. Rarity of this lesion warrants its mention.