Among primary malignant tumors of the heart, primary cardiac lymphomas are extremely rare. Early diagnosis is crucial in primary cardiac lymphoma cases as its non-specific symp- toms often lead to delayed diagnosis and poor prognosis. In this case report, we presented a challenging case of primary cardiac lymphoma that was noticed during echocardiography of a patient admitted with acute myocardial infarction. A 32-year-old man was admitted to the emergency department with acute anterior ST-elevated myocardial infarction. His angiogram revealed an acute occlusion in the proximal left anterior descending artery with otherwise normal coronary arteries. After the total occlusion was passed with a guide- wire, only a dense thrombus was observed. Therefore, an embolic source was suspected. Echocardiography revealed a giant mass (6 cm × 2.5 cm) attached to the interatrial septum. The patient was referred to early surgery for the resection of the mass. Histopathology and immunohistochemistry of the resected specimen demonstrated B cell non-Hodgkin lym- phoma. Positron emission tomography and computerized tomography showed no lymph node and organ involvement. Two weeks after surgery, he was discharged and referred to the hematology department for chemotherapy. After 6 cycles, the positron emis- sion tomography scan showed no abnormal accumulation indicating complete remission 7 months later. The clinical course of the patient was favorable for 1 and a half years. Acute myocardial infarction may be a manifestation of a rare entity such as primary cardiac lymphoma and an embolic source should always be considered. This is a case of pathologically diagnosed and successfully treated primary cardiac lymphoma.