Mature cystic teratoma (MCT) is the most common type of ovarian germ cell tumor occurring in females of reproductive age. It is typically benign, but rare malignant transformations have been reported in 1-2% of the cases. Among a wide variety of malignancies arising in MCTs, high grade lymphomas are the least common. We present a case of a 45- years -old female with a unilateral adnexal mass. Gross examination revealed a unilocular cyst with a smooth and intact capsule. The cyst lumen was filled with sebaceous material and hair. Except for a 5.0 cm Rokitansky nodule, no other nodule or papillary structures were identified. Microscopic examination revealed an array of mature tissues arising from different germ cell layers and foci of diffuse sheets of large atypical lymphoid cells. These were positive for CD-20 marker, confirming their B lymphoid series cell origin. A final diagnosis of a high grade B cell non-Hodgkin’s lymphoma arising in an ovarian MCT was made. Such cases have been known to be associated with a very poor prognosis, and there are no established criteria for their pre-operative diagnosis. Risk factors for malignant transformation in an MCT including tumor size, post menopausal status, and serum tumor markers are thus analyzed routinely to make a presumptive diagnosis. These coupled with extensive gross sampling of the tumor specimens, and a diligent histopathological examination may aid in an accurate diagnosis of a malignant neoplasm arising in MCTs.