Abstract Background Amyloidosis can affect the heart, causing arrhythmia, thromboembolic events and sudden cardiac death. Coronary sinus thrombosis is an uncommon though life-threatening condition which requires early identification and management. Case summary A 72-year old Caucasian man with recovered out-of-hospital cardiorespiratory arrest, who was diagnosed with coronary sinus thrombosis using cardiac imaging techniques. He had no history of invasive procedures and was diagnosed with cardiac amyloidosis based on an extracardiac biopsy positive for AL amyloid, with consistent clinical, echocardiographic and magnetic resonance criteria. Discussion A high frequency of intracardiac thrombosis is seen in amyloidosis. A multimodality imaging approach appears to be useful for the early diagnosis of coronary sinus thrombosis. The low specificity of the clinical signs, as well as the fast impairment of the patients result in fatal complications such as acute myocardial infarction, arrhythmia and sudden death. Early screening, particularly in high-risk patients, as well as the use of early anticoagulant therapy, could reduce the associated morbidity and mortality.