Background <p>Gestational diabetes mellitus (GDM) is a prevalent pregnancy complication influenced by dietary patterns. The Dietary Diabetes Risk Reduction Score (DDRRS), a composite index evaluating diet quality in relation to diabetes risk, may be a useful tool in understanding GDM etiology, yet its association with GDM has been underexplored.</p> Objective <p>To investigate the relationship between adherence to the DDRRS and the risk of GDM in an Iranian population.</p> Methods <p>A case-control study was conducted among 320 pregnant women in Urmia, Iran, including 150 women diagnosed with GDM and 170 age- and gestational age-matched healthy controls. Dietary intake was assessed using a validated food frequency questionnaire, and DDRRS was calculated based on intake of eight food components. Logistic regression models were used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for GDM risk across DDRRS tertiles, adjusting for potential confounders.</p> Results <p>Women with GDM had significantly lower DDRRS compared to controls (20.52 ± 4.11 vs. 21.53 ± 4.76, <i>p</i> = 0.04). In the fully adjusted model, participants in the highest tertile of DDRRS had a significantly lower risk of GDM compared to those in the lowest tertile (OR = 0.46; 95% CI: 0.24–0.85; p-trend = 0.01). Additionally, women with GDM reported higher intake of red and processed meats, sugar-sweetened beverages, and high-glycemic index foods, and lower intake of Poly unsaturated fatty acid (PUFA) / Saturated fatty acid (SFA) ratio, magnesium, zinc, and folate.</p> Conclusion <p>Greater adherence to the DDRRS was associated with a reduced risk of GDM. These findings suggest that promoting dietary patterns characterized by higher intake of fiber, nuts, and unsaturated fats and lower intake of red meats and sugary beverages may contribute to GDM prevention and maternal health improvement.</p> Clinical trial number <p>Not applicable.</p>