Surgery and platinum-based chemotherapy are highly efficacious for treating advanced ovarian cancers in humans, but their efficacy is less known in dogs. We evaluated the long-term treatment outcomes of seven dogs with malignant ovarian tumors with malignant abdominal effusion. Ovariohysterectomies (OVHs) were performed on all dogs; four had ovarian adenocarcinoma (AC) with gross dissemination in the peritoneum (two with pleural effusion), and three had a granulosa cell tumor (GCT) with no gross dissemination in the peritoneal cavity, although one showed pleural effusion. Effusion resolved after the OVH in all dogs. Six dogs (three ACs, three GCTs) received postoperative IV carboplatin therapy. Two dogs with GCT had no postoperative recurrence or metastasis, and one dog with GCT had recurrence 1811 days postoperatively. All dogs with AC developed recurrent effusion 171–584 days postoperatively, which resolved after intracavitary administration of cisplatin or carboplatin, with a subsequent disease-free interval of 155–368 days. Overall survival was longer for dogs with GCTs (822–1840 days) than for those with ACs (617–841 days). These results suggest that dogs with ovarian tumors with malignant effusion can survive relatively long after platinum-based chemotherapy in addition to OVH, with a more favorable prognosis for GCT than AC.