We present a case of a primary pericardic thymoma, revealed by a pericardic effusion. This ectopic localisation is very unusual; neoplastic pericarditis are usually due to metastasis of carcinomas or lymphomas. Sometimes, they are secondary to pericardic invasion by mediastinal malignant thymomas or a metastasis of thymomas. Our patient presented a single pericardic localisation of thymoma without any other site. We discuss ectopic localisations of thymic tissue and thymomas and the diagnostic usefulness of the MIC 2 antibody.