Background: In South Korea, nontuberculous mycobacteria (NTM) is not a notifiable disease, while the absence of a national surveillance system hampers accurate assessment of its incidence. Therefore, this study utilized National Health Insurance Service (NHIS) claims data to investigate nationwide trends in NTM occurrence over the past decade. Methods: We used NHIS claims (2010−2022) to assemble a cohort with International Classification of Diseases, 10th Revision A31 (A31.0, A31.1, A31.8, A31.9). For incidence, cases diagnosed in 2010−2011 were excluded. Incidence was estimated under three definitions: ≥2 outpatient visits or ≥1 inpatient admission with A31 during the study period; same as A, but with ≤180 days between visits; meeting B plus ≥1 antibiotic prescription within 180 days (treatment initiation). Age-standardized prevalence and incidence were calculated using the 2010 Korean population. Results: A total of 178,287 newly diagnosed NTM cases were identified from 2012 to 2022 (mean age 51.4 years; 66.8 % female). The age-standardized prevalence increased from 15.5 to 69.8 per 100,000 in 2010 to 2022. Incidence peaked in 2017 (38.9/100,000), then declined to 26.9 in 2022. Age-specific incidence of NTM infection showed distinct sex-related patterns. Among men, incidence was consistently concentrated in older adults, particularly those ≥80 years, throughout 2012−2022. In contrast, women experienced a marked epidemiologic shift beginning in 2017, with incidence in their 20s and 30s surpassing older age groups. Medical Aid beneficiaries consistently showed higher incidence rates. By region, Daejeon and Chungnam showed the greatest increase in incidence rates in 2022, compared to 2012. Conclusion NTM infection is increasing in Korea, with distinct epidemiologic patterns by sex, age, and socioeconomic status. The rising burden, especially among young women and the socioeconomically disadvantaged, warrants targeted public health strategies.