Background: The global burden of diabetes is increasing, with significant implications for young populations. However, data on trends in young-onset diabetes trends in East Asia remain limited. Moreover, the impact of socioeconomic status (SES) on the incidence and prevalence of young-onset diabetes in this region has not been fully investigated. This study aimed to assess temporal trends in the incidence and prevalence of diabetes mellitus among Korean individuals aged < 30 years, with a focus on disparities by SES. Methods: Data from the Korean National Health Insurance Service database (2008–2021) were analyzed. The annual incidence and prevalence of type 1 (T1DM) and type 2 diabetes mellitus (T2DM) were calculated, and trends were assessed using sex- and SES-adjusted rate ratios (aRRs). Results: The incidence of T1DM increased from 3.02 (95% confidence interval [CI], 2.78– 3.27) per 100,000 population in 2008 to 3.75 (95% CI, 3.45–4.06) in 2021, based on 11,392 newly diagnosed cases during the study period (mean ± standard deviation [SD] age, 16.28 ± 7.18 years; 47.27% male). Incidence was higher in females (aRR, 1.176; 95% CI, 1.124–1.232) and in the 0–14 age group. Prevalence more than doubled over the study period, rising from 21.82 (95% CI, 21.16–22.48) to 46.41 (95% CI, 45.33–47.49), based on populations ranging from 19.3 million in 2008 to 15.3 million in 2021. For T2DM, based on 98,887 incident cases (mean ± SD age, 22.96 ± 5.38 years; 57.26% male), incidence increased from 27.61 (95% CI, 26.87–28.36) to 60.45 (95% CI, 59.22–61.69). Incidence was higher in males (aRR, 1.250; 95% CI, 1.232–1.267) and in the 0–14 age group. Prevalence nearly quadrupled, increasing from 73.29 (95% CI, 72.08–74.49) to 270.37 (95% CI, 267.77–272.97), using the same denominator population as for T1DM. Both T1DM and T2DM incidence and prevalence were significantly higher among individuals with low SES. Conclusion: The rising burden of diabetes among young Koreans, particularly among socioeconomically disadvantaged groups, underscores the need for targeted public health interventions and early detection programs.