Pericardial cysts are intrathoracic lesions usually considered to be congenital and less frequently to be acquired. They are normally found incidentally upon chest radiography, computed tomography (CT)-scan or echocardiography. They are usually asymptomatic and have a benign behavior although sometimes they can cause clinical symptoms and diagnosis can be uncertain. We present, herein, the case of a 51-year-old male with a history of atrial arrhythmia resistant to transcatheter ablation therapies with an accidental finding of a pericardial cyst adherent to the lateral wall of the right atrium at the emerging superior vena cava. The patient was studied with transesophageal echocardiography, CT-scan and cardiovascular magnetic resonance with the evidence of dimensional increasing of the cyst at seriate controls. Considering this, in the absence of a definitive diagnosis and suspecting a link between the mass and the arrhythmia, the cyst was surgically removed through median sternotomy, off-pump on a beating heart. Postoperative course was unremarkable. Histopathologic examination confirmed it was a mesothelial pericardial cyst. At 12 months of follow-up the patient is doing well and he is in sinus rhythm. In conclusion, we believe that, although the majority of pericardial cysts need only radiological and clinical follow-up, surgical resection should be performed when the patient is symptomatic and when diagnosis is uncertain.