Purpose This study aimed to examine the influence of a WeChat-based dietary and exercise intervention on gestational diabetes mellitus (GDM) prevention in overweight/obese pregnant women in Beijing. Methods Overweight/obese pregnant women were recruited in the early stages of pregnancy. After screening by include and exclude standards, eligible women were randomly divided into two groups, intervention and control groups. The control group received a general advice session about pregnancy nutrition and weight management. The intervention group received three face-to-face sessions about personalized dietary and exercise intervention, with the help of WeChat as a monitoring tool to promote treatment plan adherence. At 24-28 weeks of pregnancy, GDM was diagnosed according to the International Association of Diabetes and Pregnancy Study Groups (IADPSG) criteria. Gestational weight gain (GWG), maternal and neonatal outcomes were also collected. Results This study analyzed 215 participants. At the mid-trimester, 42 (37.8%) women in the control group were diagnosed with GDM (n = 111) versus 25 (24.5%) in the intervention group (n = 104; p < 0.05). The intervention group gained 11.2 +/- 4.9 kg during the whole gestation period, with 4.9 +/- 3.1 kg-weight increment in the first 25 weeks of pregnancy, versus 13.4 +/- 5.0 kg and 6.9 +/- 3.2 kg in the first 25 weeks in the control group (between groups: p < 0.001/p = 0.002). Incidence of macrosomia was not significantly lower in the intervention group than in the control group (8/7.9% vs 11/9.9%) (p > 0.05). No significant difference was found in the rate of natural labor and occurrence of perinatal complications (e.g., preterm birth, gestational hypertension, and preeclampsia) between the groups (p > 0.05). Conclusions The WeChat-assisted dietary and exercise intervention was effective in reducing the occurrence of GDM and excessive weight gain in overweight/obese pregnant women. Disseminating knowledge of pregnancy and childbirth through social media platforms like WeChat could be an important part of antenatal care.