Granulosa cell tumour (GCT) is a benign tumour that affects a mare’s ovaries. In this report, a case of unilateral GCT in an ovary, which weighed 24 kg, of a 7-year-old warmblood mare is described, including ultrasonographic, clinical appearance and successful surgical removal of the tumour. The GCT was the cause of an acute hemoperitoneum and after a first stabilization of the mare, the GCT was removed through a ventral midline incision. Because of the enormous dimensions of the GCT, intra-abdominal partial resection of the tumor using a tenotomy knife was performed to exteriorize the ovarian pedicle. The technique can cause an intra-abdominal hemorrhage and the time between GCT resection and sealing of the respective ovarian pedicle should be as short as possible. After 3.5 months, the mare had a good cosmetic and functional outcome and was ridden for her intended use. Metastasis of GCT is rare, but not impossible. The mare therefore should be followed up for a longer period of time to investigate if metastasis develops.