Objective: To investigate the effect of dietary fiber intake during pregnancy on the prevention of gestational diabetes mellitus (GDM) in women who are overweight/obese prior to pregnancy. Methods: This randomized controlled trial was conducted in Shanghai General Hospital from June 2021 to March 2022. A total of 98 women who reported BMI≥24 kg/m2 prior to pregnancy were recruited before their 20th gestational week, and randomly (simple random allocation) assigned to the fiber supplement group (12 g of dietary fiber power twice daily) and the control group (standard prenatal care) from 20 to 24+6 gestational weeks. Both groups received nutrition education and dietary advice during the study. GDM diagnosis was performed by an oral glucose tolerance test (OGTT) at 25-28 weeks' gestation. Data are presented as means with SD, as medians with IQR, or as counts with percentages as appropriate. Comparisons were conducted using a t-test, Mann-Whitney U test, and χ2 test, respectively. Results: The incidence of GDM was significantly reduced in the fiber supplement group compared with the control group: 8.3 vs. 24.0% (χ2 = 4.40, p = 0.036). At OGTT, the mean fasting plasma glucose in the fiber supplement group was significantly lower than before the intervention (4.57 ± 0.38 mmol/L vs. 4.41 ± 0.29 mmol/L, p < 0.01) but not in the control group (4.48 ± 0.42 mmol/L vs. 4.37 ± 0.58 mmol/L, p = 0.150). Compared with the control group, the TG and TG/HDL-C ratio levels in the intervention group were significantly higher than those in the control group (2.19 ± 0.54 mmol/L vs. 2.70 ± 0.82 mmol/L and 1.19 ± 0.49 vs.1.63 ± 0.63, respectively, all P＜0.05). The body weight gain was significantly lower in the fiber supplement group than the control group (1.99 ± 1.09 kg vs. 2.53 ± 1.20kg, p = 0.022). None of the women randomized to the fiber supplement group experienced preterm birth (<37 weeks gestation) compared with 12.0% in the control group (p = 0.040). Excessive weight gain (total weight gain >11.5 kg for overweight, and >9.0 kg for obesity) occurred in 46.7% of women in the fiber supplement group compared with 68.0% in the control group (p = 0.035). There were no differences in other maternal and neonatal outcomes. Conclusion: Increased dietary fiber intake in pregnant women who were overweight/obese prior to pregnancy may reduce the risk of GDM, excessive weight gain, and preterm birth, but it did not improve blood lipids.