Abstract Background: Plasmablastic lymphoma (PBL) is relatively new clinical entity described as a distinct subtype of diffuse large B-cell lymphoma (DLBCL). It is characterized by its aggressive nature and proliferation of large neoplastic cells resembling immunoblasts including cells with more obvious plasmacytic differentiation. In this case report, we describe an unexpected finding of PBL associated with a mature cystic teratoma of the ovary in a young immune competent woman. Case presentation: A 19-year old woman was admitted to the hospital with generalized lymphadenopathy, a pelvic tumor mass measuring 35 × 30 cm and a 4 cm lump in her right breast. She underwent a right salpingo-oophorectomy, lymphadenectomy, splenectomy, omentectomy, and a right breast lumpectomy. On macroscopic examination the right ovary was replaced by a thick-walled multilocular cystic tumor. Upon incision, the cysts were filled with thick, greasy sebaceous material and hair and there were several solid nodules within the cyst walls. Histological examination revealed a mature cystic teratoma and malignant non- Hodgkin lymphoma (NHL) within the solid nodules. Tumor tissue from the right breast, spleen and lymph nodes, all had the same histological, NHL morphology. After extensive immunostaining, a diagnosis of PBL was made. Following surgery, the patient was treated with different chemotherapy regimens, without any significant regression of the disease, and died of multiple organ failure. Conclusions: Primary NHL of the ovary is relatively rare occurrence while secondary involvement by lymphoma is much more common. PBL is a rare lymphoma, primarily reported in the jaw and oral mucosa, but also documented in extra-oral sites. To the best of our knowledge, this is the first case described in a mature ovarian cystic teratoma. Although the patient was HIV-negative and immune competent, she had progressive disease and died despite aggressive chemotherapy 11 months after the initial diagnosis.