MODY3 is one of the most common subtypes of MODY. Obesity in MODY3 patients modifies the disease course and complicates diagnostics at the clinical stage. A proband was diagnosed with type 2 diabetes mellitus (T2DM) at the age of 12 years; metformin therapy was used. A family history of DM involves three generations: the mother, aunt, and maternal grandfather have suffered from insulin-dependent DM since the age of 23, 22, and 40 years, respectively. The patient was examined at the age of 14 years. Obesity was present (SDS BMI 2.3). The insulin and C-peptide levels were 4.4 μU/mL and 1.5 ng/mL, respectively. The HbA1c level was 7.3%. Under glucose load, glycemia reached diabetic values; hyperinsulinemia and insulin resistance were not detected. Specific pancreatic antibodies were absent. Metformin was discontinued, and a sulfonylurea (SU) drug was prescribed, which had a positive effect. The heterozygous mutation p.P291fs was identified in the HNF1A gene. Therefore, MODY3 was verified. The presence of concomitant obesity in the patient significantly complicates the differential diagnosis, and only a careful comprehensive analysis of clinical and laboratory parameters and a family history makes it possible to suspect the diagnosis of MODY3 (requiring subsequent molecular genetic verification) and prescribe pathogenetic therapy.