AIMS: The effectiveness of physical activity (PA) programs for prevention of gestational diabetes (GDM) lacks conclusive evidence. The aim of this study was to generate clear evidence regarding the effectiveness of physical activity programs in GDM prevention to guide clinical practice. METHODS: PubMed/Medline, ISI Web of Science, Scopus, and EMBASE were searched to identify the randomized trials (RCTs) published until June 2019. Randomised controlled trials enrolling women at high risk before the 20th week of gestation comparing the effect of PA interventions with usual care for prevention of GDM were retrieved. Data obtained were synthesised using a bias-adjusted model of meta-analysis. RESULTS: A total of 1467 adult women in 11 eligible trials were included. The risk of GDM was significantly lower with PA, but only when it was delivered in the healthcare facility (RR 0.53; 95% CI 0.38 to 0.74). The number needed to treat with PA in pregnancy (compared to usual care) to prevent one GDM event was 18 (95% CI 14 - 29). The overall effect of PA interventions regardless of location of the intervention was RR 0.69 (95% CI 0.51 - 0.94). CONCLUSIONS: This study provides evidence that in-facility physical activity programs started before the 20th week of gestation can significantly decrease the incidence of GDM among women at high risk.