A 41-year-old woman with history of hypoglycemic symptoms underwent sleeve gastrectomy for weight loss. Her postoperative course was complicated by cardiac arrest due to polymorphic ventricular tachycardia arising from QT interval prolongation. Weeks after surgery, hypoglycemic symptoms worsened, with postprandial capillary glucose as low as 33 mg/dL (SI: 1.2 mmol/L) (reference, 70-140 mg/dL [SI: 3.9-7.8 mmol/L]). An inpatient fast indicated appropriate suppression of insulin. The combination of worsening hypoglycemia early postoperatively and prolonged QT interval prompted genetic testing, which revealed a likely pathogenic variant in KCNE1, encoding a subunit of the voltage-gated potassium channel Kv7.1, expressed in cardiomyocytes and pancreatic β cells. The patient was treated with nutrition therapy and diazoxide for hypoglycemia and β-blocker for long QT syndrome (LQTS). Patients with KCNE1 pathogenic variants are known to have LQTS and have been reported to have postprandial hypoglycemia. This case highlights the importance of considering genetic etiologies when encountering concomitant hypoglycemia and LQTS, and that preexisting LQTS may confer risk for hypoglycemia after upper gastrointestinal surgery.