We report the case of a primary serous ovarian borderline tumour developed in an inguinal lymph node. No primary ovarian borderline tumour was observed within the ovaries after bilateral ovariectomy and complete pathological examination. We considered the diagnosis of ectopic ovarian tissue because the tumour was completely surrounded by normal ovarian tissue with positive FOXL2 staining. The whole ovarian tissue was itself entirely surrounded by lymphatic tissue. Two other hypotheses should be considered: primary retroperitoneal borderline tumour or retroperitoneal nodal involvement by an ovarian serous borderline tumour. Ectopic ovarian tissue can be observed in lymph node. We don't believe ectopic location of ovarian tissue increase the risk of neoplastic change.