A 37-year-old female yellow-naped Amazon parrot (Amazona auropalliata) was presented with a history of lethargy, inappetence, and decreased vocalizations. On examination, the coelom was moderately distended and palpated fluctuant, and the heart was muffled on auscultation. Coelomic ultrasound, coelomocentesis, and radiographs were performed and revealed an enlarged cardiac silhouette and marked coelomic effusion. Pericardial effusion was confirmed by echocardiography. A well-circumscribed, hyperechoic soft tissue density was observed at the level of the right atrium on initial echocardiography; however, a cardiac mass was not identified by computed tomography scan or repeat echocardiograms. Ultrasound-guided pericardiocentesis was performed under anesthesia, and cytology results were consistent with hemorrhage; no neoplastic cells were identified. A repeat echocardiogram 4 days after pericardiocentesis revealed recurrence of the pericardial effusion. Due to the grave prognosis, the owners declined endoscopic pericardiectomy, and the patient died the following day. On postmortem examination, the pericardial surface of the heart was covered in a white to yellow, multinodular mass layer. Histologic analysis revealed a multinodular mass extending from the atria, running along the epicardium distally, and often extending into the myocardium. Neoplastic cells present in the heart mass and pericardium did not stain with a Churukian-Schenk stain, and thyroglobulin immunohistochemistry was negative. Cytokeratin and vimentin stains showed positive expression in the neoplastic cells within the mass. These results are consistent with a diagnosis of mesothelioma. This is the first report of mesothelioma in a psittacine bird.