Summary: A rare case of bilateral perineal hernia was reported in a 24-year-old Italian Trotter mare. This condition is described as a protrusion of retroperitoneal fat, and/or the abdominal or pelvic viscera through the pelvic diaphragm which supports the rectal wall. It is commonly observed in olduncastrated old male dogs, but occurs very rarely in large ruminants; to the Authors’ knowledge, it has never been described in horses. Chronic weight loss is the main clinical sign, even if the main concern regards the entrapment and the strangulation of the bowel content. Reaching a diagnosis involves clinical signs, palpation, ultrasound evaluation and rectal examination. The approach chosen in all the cases of perineal hernia described in large ruminants is appositional herniorrhaphy; for this reason, it was utilised as the first surgical choice in the present case. Due to its failure, a new surgical approach was used The second procedure involved the transposition of the semimembranosus muscle. The short-term outcome after the second surgery presented no recurrence or complications. It is therefore necessary to consider perineal hernia among the various differential diagnosis of perineal swellings in horses. Additional cases are required to determine its benefits.