The morphology of ovarian tumors is characterized by a variety of histological features causing diagnostic difficulties. In human medicine, HBME-1 (Hector Battifora mesothelial epitope)-1 is one of the immunohistochemical markers employed in the diagnosis of ovarian epithelial tumors. The aim of the current study was to investigate the reliability of the marker HBME-1 in canine ovaries, granulosa cell tumors, and epithelial ovarian neoplasms to determine whether this marker could be included in an immunohistochemical panel for differential diagnoses of canine ovarian tumors. Samples were obtained from 4 normal ovaries, 10 granulosa cell tumors, and 18 epithelial ovarian tumors. After formalin fixation and paraffin embedding, tissue sections were stained with hematoxylin and eosin and probed immunohistochemically for the HBME-1 marker. Granulosa cells and related tumors were consistently negative for HBME-1. Normal ovarian surface epithelium and 17 out of 18 ovarian epithelial tumors were positive for HBME-1. The results suggested that HBME-1 would be a useful marker for the differential diagnosis of ovarian tumors in the dog.