Congenital hyperinsulinism (CHI) is a rare hereditary condition that causes excessive insulin production in the clinical picture of severe hypoglycemia. Patients with CHI frequently have mutations in the ABCC8 and KCNJ11 genes, which code for KATP channels in pancreatic beta cells. We present a case of partial diazoxide responsiveness caused by a heterozygous ABCC8 mutation in a child with moderate CHI. A four-month-old term baby with a birth weight of 4.8 kg came with seizures and hypoglycemia. A hypoglycemia panel test verified CHI. Diazoxide was started due to persistently high glucose needs (10 mg/kg/min), but after escalation to a high dose (14 mg/kg/day), divided every 6 hours, titrated timing; glucose-enriched formula meals, glucose levels remained below safe range in the late afternoon. Diazoxide medication was thus continued, and a once-daily subcutaneous octreotide infusion was started. Both intravenous glucose and glucagon were terminated as normoglycemia was established throughout the day at a dosage of 8mg/kg/day of diazoxide and 2 mg/kg/dose once a day of octreotide. Later genetic testing revealed a heterozygous ABCC8 missense mutation [NM 001287174.1c:89>T, p.Ala30Val]. The mutation is a paternally inherited monoallelic variant previously linked to a focused type of CHI. This patient was previously assumed to be diazoxide non-responsive, but after a timed low dosage of octreotide and a lower dose of diazoxide, he achieved a near-optimal glycemic response. We emphasize the clinical manifestation of this missense heterozygous variant in a patient with CHI in our report to contribute to the strengthening of the data on this likely-pathogenic variant.