Purpose: Type 1 diabetes mellitus (DM) is the most common cause of diabetes in children and adolescents. Prevalence of monogenic DM is estimated for about 1%–5% of all patients with DM. Overlapping clinical features of various forms of diabetes make differential diagnosis challenging. This study was performed to investigate frequency and genetic etiologies of monogenic diabetes in a single academic center. Methods: This study included 466 consecutive patients with DM diagnosed before 18 years of age from January 1996 to July 2017. Clinical features, biochemical findings, β-cell autoantibodies, and molecular characteristics were reviewed retrospectively. Results: Three hundred and thirty two patients (71.2%) had type 1 DM, while 108 patients (23.2%) were type 2 DM. Genetic etiologies were identified in the remaining 26 patients (5.6%). Three patients diagnosed with maturity onset diabetes of the young with mutations in HNF1A (p.G292Rfs*26) and HNF1B (p.S148L, p.A166P). Two siblings manifested bilateral optic atrophy and urinary incontinence at adolescent period and were diagnosed with Wolfram syndrome caused by WFS1 mutation. A male with acanthosis nigricans, hirsutism, high insulin level, and intrauterine growth retardation was compound heterozygote for INSR (p.R1066*/p.Q1232*). IPEX syndrome was found in a male who presented membranous glomerulopathy, pure red cell aplasia, and posterior reversible encephalopathy syndrome. Eight patients were diagnosed with neonatal DM: two with transient form caused by paternal uniparental disomy of 6q24 and the other six with permanent form with mutations in KATP channel genes, including a male DEND syndrome. A 3-month-old Arab girl presented with diabetes and liver failure, and were diagnosed with Wolcott-Rallison syndrome caused by EIF2AK3 p.W431* mutation. Two patients CFTR mutations displayed diabetes with associated features such as pancreatitis and recurrent infections. Eight patients with MELAS syndrome were classified as mitochondrial DM at age 27.3 ± 11.3 years with the HbA1c level of 6.6 ± 1.8%. Conclusions: It should be considered that diabetic patients who had family history or extra-pancreatic features without β-cell autoantibodies might have monogenic diabetes. Identification of the genetic cause of DM is critical to provide appropriate therapeutic options and genetic counselling.