Vitamin B12 deficiency has been associated with a plethora of metabolic abnormalities, such as hyperhomocysteinaemia, insulin resistance and defective synthesis of neurotransmitters and fatty acids. Inconsistency exists as to whether vitamin B12 deficiency is also associated with increased risk of gestational diabetes mellitus (GDM). The purpose of this study was to systematically review and meta-analyze the existing evidence for this association.A comprehensive search was conducted in PubMed, Scopus and Cochrane Central up to April 30, 2019. Data are expressed as odds ratio (OR) with 95% confidence interval (CI). The I index was employed for heterogeneity.Six studies (n = 1810 pregnant women, 309 GDM cases) fulfilled the eligibility criteria for qualitative and two studies for quantitative analysis. In five studies providing data on vitamin B12 concentrations for both groups, women with GDM had lower vitamin B12 levels when compared with non-GDM women. Women with vitamin B12 deficiency were at higher risk for developing GDM when compared with those who were vitamin B12 sufficient: OR 1.81 (95% CI, 1.25-2.63, I: 0%). Due to the small number of studies, the role of potential confounders could not be safely estimated.Vitamin B12 deficiency seems to be associated with increased risk of GDM. More studies are needed to further strengthen this finding and to clarify possible pathogenetic mechanisms.