A 15-year-old gelding presented with a progressive lameness of the left forelimb of 2.5 months duration. Clinically, a dilation of the deep flexor tendon sheath with a firm elastic consistency and a pronounced tenderness was noted. Ultrasonically, a marked swelling of the flexor tendon sheath with an irregular density of the mesotendineum was observed. The white, firm material forming a nodular distension of the flexor tendon sheath with a diameter of approximately 1 cm was excised and sent for histopathological examination. Biopsies of the deep flexor tendon and corresponding tendon sheath were sent for histopathological evaluation. Histologically, the mass consisted of clefts and numerous anastomosing vascular channels extending between the collagen fibers of the deep flexor tendon. These capillary-like spaces were lined by neoplastic cells that were flattened to polygonal and contained few erythrocytes. There was 0 to 1 mitotic figure per 10 high power fields (400×). Immunohistochemically, the neoplastic cells stained positive for vimentin and factor VIII-related antigen. Adjacent to the neoplastic endothelial cells located pericytes expressed α-smooth muscle actin antigen. Based on the histopathological and immunohistochemical features, synovial hemangioma was diagnosed. One year after surgery, the horse has shown no lameness.