Maturity-Onset Diabetes of the Young (MODY) is a heterogeneous group of diseases associated with genes mutations leading to dysfunction of pancreatic β-cells. Among the 14 identified MODY variants, MODY 1–5 are the most studied. The article reports a MODY 12 clinical case, with mutation in ABCC8, encoding the sulphonylurea receptor. Diabetes mellitus manifested in a 27-year-old man with hyperglycaemia up to 24 mmol/L, without ketosis. Non-proliferative diabetic retinopathy, microalbuminuria, dyslipidaemia and carotid atherosclerosis were revealed upon initial examination. The levels of pancreatic islet cell antibodies and glutamate decarboxylase antibodies were negative, while the level of C-peptide was within the normal range. Insulin therapy in the basal-bolus regimen was provided with a gradual dose reduction due to frequent hypoglycaemia. The preproliferative retinopathy with macular oedema was revealed after 4 months of therapy, and panretinal photocoagulation of both eyes was performed. A molecular genetics study revealed a mutation in the gene ABCC8, the same mutation was found in patient’s mother and uncle. Insulin therapy was cancelled, and the treatment of gliclazide MR 60 mg/day was initiated, which resulted in extreme glycaemic excursions. Thereby, sodium–glucose cotranporter-2 (SGLT2) inhibitor dapagliflozin 10 mg/day was added. A reduction in glucose variability parameters were observed on combination therapy. After 6 months till 1.5 years of treatment, glycaemic control was optimal, no hypoglycaemic episodes were observed. This case study demonstrates clinical features of MODY 12, and the potential of combination of sulfonylurea and SGLT2 inhibitor in the treatment of this disease.