The authors describe a case of bilateral diffuse paraneoplastic orbital myositis induced by a stage IA left testicular pure seminoma. The patient presented with findings typical of thyroid-associated orbitopathy (TAO) and was thought to have TAO until discovery of the malignancy. Treatment included an urgent orchiectomy, as well as 7 weeks of therapeutic plasma exchange. This is the fifth reported case of seminoma-associated orbitopathy, and the second to occur while cancer was in the occult phase. Although seminoma-associated orbitopathy is exceedingly rare, it can masquerade as TAO and should be considered in the differential diagnosis of any young male with atypical TAO findings.