OBJECTIVE Dietary interventions can improve pregnancy outcomes in women with gestational diabetes mellitus (GDM). We compared the effect of a low–glycemic index (GI) versus a conventional high-fiber (HF) diet on pregnancy outcomes, birth weight z score, and maternal metabolic profile in women at high risk of GDM. RESEARCH DESIGN AND METHODS One hundred thirty-nine women [mean (SD) age 34.7 (0.4) years and prepregnancy BMI 25.2 (0.5) kg/m2] were randomly assigned to a low-GI (LGI) diet (n = 72; target GI ∼50) or a high-fiber, moderate-GI (HF) diet (n = 67; target GI ∼60) at 14–20 weeks’ gestation. Diet was assessed by 3-day food records and infant body composition by air-displacement plethysmography, and pregnancy outcomes were assessed from medical records. RESULTS The LGI group achieved a lower GI than the HF group [mean (SD) 50 (5) vs. 58 (5); P < 0.001]. There were no differences in glycosylated hemoglobin, fructosamine, or lipids at 36 weeks or differences in birth weight [LGI 3.4 (0.4) kg vs. HF 3.4 (0.5) kg; P = 0.514], birth weight z score [LGI 0.31 (0.90) vs. HF 0.24 (1.07); P = 0.697], ponderal index [LGI 2.71 (0.22) vs. HF 2.69 (0.23) kg/m3; P = 0.672], birth weight centile [LGI 46.2 (25.4) vs. HF 41.8 (25.6); P = 0.330], % fat mass [LGI 10 (4) vs. HF 10 (4); P = 0.789], or incidence of GDM. CONCLUSIONS In intensively monitored women at risk for GDM, a low-GI diet and a healthy diet produce similar pregnancy outcomes.