Acute T-cell lymphoblastic lymphoma is a rare subtype of non-Hodgkin’s lymphomaaccounting for 2% of adult non-Hodgkin’s lymphomas. Cardiac involvement in non-Hodgkin’slymphoma is usually a late manifestation, and pericardial effusion as an initial presentationis rare. We report a case of acute T-cell lymphoblastic lymphoma in a patient who initiallypresented with pericardial effusion. Diagnosis was difficult and challenging, as an infectiousetiology was first suspected. The patient presented with pericardial effusion without othercommon clinical manifestations of lymphoma, such as fever, night sweats, and weight loss. Hewas treated for constrictive pericarditis due to having positive Coxsackie B serology but hadno improvement with treatment. The pathology results of the pericardium after pericardiectomyreported fibrosis with a reactive lymphohistiocyte infiltrate. After CT scan revealed a largemediastinal mass, a lymph node biopsy was performed, and T-cell lymphoblastic lymphomawas confirmed. A detailed evaluation to search for occult malignancy should be considered inpatients who have persistent pericarditis that is unresponsive to anti-inflammatory therapy andin those who present with a new, large pericardial effusion or cardiac tamponade.