This systematic review and meta-analysis evaluated the effects of vitamin D supplementation on glucose metabolism and pregnancy outcomes in patients with gestational diabetes mellitus (GDM). It included 20 studies with 1737 patients. Vitamin D supplementation significantly reduced fasting glucose (SMD: −1.01, p=0.0002), 2-hour postprandial glucose (SMD: −0.89, p=0.0002), insulin levels (SMD: −0.64, p<0.0001) and insulin resistance (p=0.01). It was associated with a lower incidence of caesarean section (RR: 0.68, p<0.0001), forceps-assisted delivery (RR: 0.44, p=0.02), preterm delivery (RR: 0.28, p<0.0001), postpartum hemorrhage (RR: 0.27, p=0.004), neonatal asphyxia (RR: 0.22, p=0.004). p=0.006), macrosomia (RR: 0.34, p=0.001) and neonatal hyperbilirubinemia (RR: 0.49, p=0.001). No significant differences were found for amniotic fluid excess (RR: 0.46, p=0.10) or preeclampsia (RR: 0.60, p=0.38). The conclusion states that supplementation may improve glucose metabolism and reduce adverse pregnancy outcomes, but with caution due to the heterogeneity and methodological limitations of the studies.