Sepsis of the temporomandibular joint (TMJ) of the horse is rare. This case report describes the use of radiography, ultrasonography and computed tomography to diagnose sepsis of the TMJ in a 2-year-old Thoroughbred filly. The mandibular condyle and temporal bone were partially resected to successfully remove necrotic tissue within and surrounding the TMJ. Sepsis resolved and the horse returned to race training. Follow-up computed tomography examination showed generation of a mandibular ‘pseudocondyle’ at 8 months post operatively.