Summary: Thoracic vertebral malformations occur frequently in small animals; however, they are uncommon in horses and usually appear as incidental findings on radiographs or at necropsy. Very few affected horses show clinical signs and typically these are observed at a young age. The previously reported cases of clinically apparent hemivertebrae in the thoracic spine of horses describe animals less than 28 months old. This case report describes the clinical, diagnostic and post-mortem findings in a 14-year-old gelding with a thoracic hemivertebra and insidious onset of ataxia. In addition to being the first reported clinically apparent case of a thoracic hemivertebra in an adult horse, this case illustrates the difficulty of diagnosing thoracic vertebral malformations in mature horses. Computed tomography of the thoracic spine can yield a reliable post-mortem diagnosis.