Objective: Air pollution is hypothesized to be a risk factor for gestational diabetes mellitus (GDM). Epidemiological evidence is inconsistent and has not been systematically evaluated. We systematically reviewed epidemiological evidence on the association between air pollution and GDM. Methods: We systematically searched electronic literature databases (last search, 30 July 2018) for studies reporting the association between air pollution (particle concentration and gaseous pollutants) and GDM. We systematically evaluated risk of bias and role of potential confounders in all studies. Results: We included 8 studies, 2 conducted in Europe, 4 in United States and 2 in Taiwan. Seven studies were longitudinal and 1 case-control. Four studies included both ambient particulate matter (PM) and gaseous pollutants exposure, 2 only PM exposure, and 2 only NOx exposure. Only one study examined effects on both impaired glucose tolerance (IGT) and GDM, and increased risk of IGT by odds ratio 1.64 per IQR (95% CI 1.11- 2.42). Risk of bias, air pollution assessment, and confounder control varied across studies. Two studies on PM2.5 (PM ≤ 2.5 μm in diameter) showed increased risk of GDM by odd ratio 1.07-1.20 (95% CI 1.01-1.26) per IQR during preconception, different trimesters or full pregnancy. And three studies showed no significant association. Three studies on SO2 (sulfur dioxide) showed increased risk of GDM by odd ratio 1.05-1.38 (95% CI 1.00-1.46) per IQR. Four studies on NO, NO2, or NOx showed increased risk of GDM by odd ratio 1.01-1.24 (95% CI 1.00-1.49) per IQR and only one study showed no association. Associations were stronger in first and second trimesters, but not consistent. Conclusion: Existing evidence indicates almost a positive association of air pollution and GDM risk. This finding may have implications for pregnant women to reduce GDM risk. However, different country and environment cause inconsistent result, more researches about this issue is needed.