A rare case of bilateral perineal hernia has been reported in a 24-years-old Italian Trotter mare. This condition is described as a protrusion of retroperitoneal fat, abdominal or pelvic viscera through the pelvic diaphragm which supports the rectal wall. It is commonly recorded in uncastrated old male dogs, occurs very rarely in large ruminants and, to the author’s knowledge, it has never been described in horses. Chronic weight loss was the main clinical signs, even if the main concerns regarded the entrapment and the strangulation of the bowel content. Palpation demonstrated that the swelling was painless, soft and reducible. Diagnosis consists in clinical signs, palpation, ultrasound evaluation and rectal examination. The chosen approach in all the described cases of perineal hernia in large ruminants was the appositional herniorrhaphy; for that reason it has been used as the first surgical choice in our case. Due to its failure, a new surgical approach was carried out. 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 and to take in account the muscle transposition technique as a valid surgical alternative. Further cases are required to determine its benefits.