Lipomatous hypertrophy of the heart is a benign rare tumor characterized by fatty tissue deposits in the interatrial septum. Its diagnosis during life is mostly seen incidental. We report the case of a 74-year-old female patient, in which a cardiac mass protruding in the right atrium was diagnosed by transthoracic echocardiography. Transoesophageal echocardiography examination detected an echogenic infiltration and hypertrophy of the interatrial septum, sparing the foramen ovale. The diagnosis of lipomatous hypertrophy of the interatrial septum is confirmed by MRI, which shows a well limited and not encapsulated tumor, appearing in increased signal density on T1-weighted images, and that decreases with fat-saturated sequences. This tumor did not enhance with the administration of contrast material. The benign and lipomatous nature of the lesion is confirmed, thus making the use of histology unnecessary.