Primary ovarian lymphoma is extremely rare, and such a case is reported here. The patient was a 54-year-old Japanese female with abnormal genital bleeding. Pelvic CT and MRI showed a right ovarian tumor with a diameter of 7 cm and an irregular border. With the diagnosis of a right ovarian tumor, the patient underwent a simple hysterectomy and bilateral salpingo-oophorectomy. Microscopic examination of the right ovarian tumor revealed vaguely nodular growth of small lymphoid cells. They were CD10+, Bcl-2+, Cyclin D1- and CD21-, although CD21+ follicular dendritic cell clusters were present as a background component in each vague nodule. A conventional cytogenetic analysis revealed t(14;18)(q32;q21), and somatic hypermutation of the variable region of the immunoglobulin heavy chain gene (IgH) was confirmed by direct sequencing of subcloned DNA samples derived from PCR amplicons. These findings led us to characterize the lesion as follicular lymphoma, grade 1. The patient was free of detectable disease 9 months after initiation of post-surgical chemotherapy. Since the prognosis for primary ovarian lymphoma is relatively favorable in many cases, it is important to establish therapeutic methods for the cure of this disease using chemotherapy and radiotherapy without radical surgery.