Background <p>Hypercalcemia caused by non-Hodgkin B-cell lymphomas is not common, and it rarely manifests as the first presentation for medical attention. Few case reports describe atrial fibrillation as the presentation of parathyroid hormone-dependent hypercalcemia. However, the medical literature has not previously reported a non-Hodgkin lymphoma patient presenting with atrial fibrillation due to severe hypercalcemia.</p> Case presentation <p>We present the case of a 50-year-old, previously healthy Sri Lankan woman who experienced palpitations for a single day. Upon examination, we discovered supraclavicular lymphadenopathy along with epigastric tenderness and fullness. We found her to have stable, fast atrial fibrillation, and further evaluation revealed severe parathyroid hormone-independent hypercalcemia. A supraclavicular lymph node biopsy revealed a diagnosis of high-grade diffuse large B-cell lymphoma. Atrial fibrillation was converted to a complete heart block after the administration of digoxin in this patient on presentation to the emergency unit, which was corrected after hypercalcemia correction.</p> Conclusion <p>Hypercalcemia can rarely have cardiac involvement, including conduction defects. It is important to assess the calcium level in a patient with cardiac arrhythmias because hypercalcemia is treatable.In a patient with hypercalcemia, it is advisable to avoid digoxin for rate control.</p>