Background <p>Maturity-onset diabetes of the young (MODY) is a clinically and genetically heterogeneous group of monogenic diabetes subtypes that is frequently misdiagnosed as type 1 or type 2 diabetes, and accurate genetic diagnosis enables a precision-medicine treatment approach. Regional Turkish data are limited, and previous Turkish series have been pediatric only. We characterized the genetic spectrum and therapeutic consequences of next-generation sequencing (NGS)-based MODY testing in a combined adult-and-pediatric cohort from the eastern Black Sea region of Türkiye.</p> Methods and results <p>We retrospectively analyzed 296 consecutive patients with clinically suspected MODY referred between January 2022 and June 2025. A targeted NGS panel covering 14 MODY genes was applied, variants were classified per 2015 ACMG/AMP criteria, and treatment changes attributable to the genetic diagnosis were extracted from medical records. Pathogenic or likely pathogenic (P/LP) variants were identified in 43 of 296 patients (14.5%); diagnostic yield rose to 26.0% with variants of uncertain significance included. <i>GCK</i>-MODY accounted for 72.1% of P/LP findings, with a recurrent frameshift c.1256del p.(Phe419SerfsTer12) across nine apparently unrelated families consistent with a regional founder allele. Rare subtypes included MODY4 (<i>PDX1</i>), MODY6 (<i>NEUROD1</i>), MODY8 (<i>CEL</i>), MODY10 (<i>INS</i>), MODY12 (<i>ABCC8</i>) and MODY13 (<i>KCNJ11</i>). The genetic diagnosis directly modified pharmacological therapy in 14 patients, including insulin discontinuation in three K<i>ATP</i>-channel MODY and one <i>INS</i>-MODY case.</p> Conclusions <p>NGS-based MODY testing yields actionable findings in approximately one in seven clinically selected patients in this region and supports inclusion of MODY testing in routine endocrinology practice.</p>