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Background/Objectives: This population-based observational study with a repeated cross-sectional design included singleton live births occurring in Tuscany, Italy, between 2010 and 2021, identified through regional Delivery Assistance Certificates (CeDAP) and linked with administrative healthcare databases. The study aimed to assess temporal trends in the prevalence of gestational diabetes mellitus (GDM) and to evaluate the contribution of major risk factors-particularly pregestational obesity, maternal age, parity, and origin from high-pressure migration countries (HPMC)-to GDM and related maternal-neonatal outcomes. The rationale for the study derives from the increasing worldwide prevalence of GDM, driven by changes in diagnostic criteria as well as evolving maternal demographic and metabolic risk profiles. Methods: The study included 266,394 singleton live births. GDM was identified using a previously validated algorithm based on insulin prescriptions, diabetes specialist visits, attendance to education programs, and postpartum oral glucose tolerance testing. Temporal trends in GDM, pregestational obesity, HPMC origin, and maternal-neonatal outcomes were assessed. Multivariable Poisson regression models with robust variance estimation were used to estimate adjusted prevalence ratios (PRs) and 95% confidence intervals (CIs) for GDM and adverse outcomes, including preterm birth, macrosomia, neonatal distress, and cesarean delivery. Results: During the study period, the annual number of births declined, whereas the overall mean prevalence of GDM was 11.37% and increased steadily from 8.23% in 2010 to 14.43% in 2021, corresponding to a relative increase of approximately 75%. Pregestational obesity and pregnancies among women from HPMC also increased over time. Multivariable analysis showed that pregestational obesity, advanced maternal age, multiparity, and HPMC origin were major independent determinants of GDM. Pregestational obesity emerged as the most important variable associated with both GDM and maternal-neonatal complications, including macrosomia, cesarean delivery, and neonatal distress, often showing stronger associations than GDM itself. Macrosomia and cesarean delivery declined over time, whereas preterm birth increased among pregnancies complicated by GDM. An inverse association between GDM and fetal macrosomia was observed in adjusted models, possibly reflecting effective clinical management, earlier delivery strategies, or algorithm-related selection effects. Conclusions: GDM prevalence increased substantially in Tuscany between 2010 and 2021 despite declining birth rates. This increase appears to be mainly associated with rising pregestational obesity, older maternal age, multiparity, and changes in population composition related to migration from HPMC. Adverse maternal-neonatal outcomes were more strongly associated with pregestational obesity and maternal age than with GDM itself, suggesting that preventive strategies targeting metabolic health before pregnancy may be crucial for reducing both GDM and related complications. Monitoring temporal trends in GDM and its risk factors may also serve as an indirect indicator of the effectiveness of regional healthcare policies.