The primary aim of this systematic review and meta-analysis was to determine the association between gestational diabetes mellitus (GDM) and metabolic syndrome (MetS) in women and children. Our secondary aim was to assess the development of MetS with respect to the elapsed time postpartum at which MetS was diagnosed. This review is registered with PROSPERO (CRD42020173319). PubMed, CINHAL, SCOPUS, and EMBASE databases were searched. Studies reporting on the rate of MetS in pregnant women with GDM, the rate of MetS in women with a history of GDM, and the rate of MetS in offspring exposed to GDM in utero compared to healthy controls were selected. We identified 588 articles from the literature search. Fifty-one studies were included in the review and of those 35 were included in the meta-analysis. Quantitative summary measures showed that women with a history of GDM had an increased risk of developing MetS compared to those without a history of GDM (RR 2.36, 95% CI 1.77-3.14, 29 studies, 13,390 participants; heterogeneity: χ2 p < 0.00001; I2 = 93%). Offspring exposed to GDM in utero have an increased risk of developing MetS compared to those not exposed to GDM in utero. (RR 2.07, 95% CI 1.26-3.42, three studies, 4,421 participants; heterogeneity: χ2 p = 0.33; I2 = 12%). Women diagnosed with GDM have an increased risk of developing MetS during pregnancy (RR 20.51, 95% CI 5.04-83.55; three studies, 406 participants; heterogeneity: χ2 p = 0.96; I2 = 0%). Subgroup analysis revealed that MetS is diagnosed as early as <1 year postpartum in women with a history of GDM. Women with GDM have an increased risk of developing MetS during pregnancy. Women with a history of GDM and offspring exposed to GDM in utero have higher risks of developing MetS compared to those with no history of GDM. Metabolic syndrome in women with a history of GDM is seen as early as <1 year postpartum.