Granulosa theca cell tumors (GTCTs) are the most common neoplasia of the equine ovary, comprising about 2.5% of all equine neoplasms. Historical facts include reports of failure to cycle, unusual sexual behavior (nymphomania or stallion like), or a change in sexual behavior. The present study reports a case of a 10-year-old crossbreed mare with GTCT presenting nymphomania behavior (continuous estrus) and no pregnancy. This tumor was diagnosed by clinical history, rectal palpation and ultrasonography before surgical removal. Histopathological examination confirmed the diagnosis by showing irregular aggregates of neoplastic granulosa resembling cells. Serological levels of testosterone and inhibin were note evaluated. The mare recovered and by one month and 15 days was evacuated out with other horses. To our knowledge, this is the first report of Granulosa theca cell tumors in a mare in Algeria. The only form of treatment is surgical removal of the affected ovary that will eliminate the behavioral changes and allow recrudescence of cyclicity of the remaining ovary.