South Asian women are at increased risk of developing gestational diabetes (GDM). Few studies have investigated the genetic contributions to GDM risk. We in vestigated the association of a type 2 diabetes (T2D) polygenic risk score (PRS), on its own, and with GDM risk factors, on GDM-related traits using data from two birth cohorts in which South Asian women were enrolled during pregnancy . 837 and 4,372 pregnant South Asian women from the South Asian BiRth CohorT (START) and Born in Bradford (BiB) cohort studies underwent a 75-gram glucose tolerance test. PRSs were derived using GWAS results from an independent multi-ethnic study (~18% South Asians). Associations with fasting plasma glucose (FPG); 2h post-load glucose (2hG); area under the curve glucose; and GDM were tested using linear and logistic regressions. The population attributable fraction (PAF) of the PRS was calculated. Every 1 SD increase in the PRS was associated with a 0.085 mmol/L increase in FPG ([95%CI=0.07-0.10], P=2.85 × 10-20); 0.21 mmol/L increase in 2hG ([95%CI=0.16-0.26], P=5.49 × 10-16); and a 45% increase in the risk of GDM ([95%CI=32-60%], P=2.27 × 10-14), independent of parental history of diabetes and other GDM risk factors. PRS tertile 3 accounted for 12.5% of the population’s GDM alone, and 21.7% when combined with family history. A few weak PRS and GDM risk factors interactions modulating FPG and GDM were observed. Together, these results show that a T2D PRS and family history of diabetes are strongly and independently associated with multiple GDM-related traits in women of South Asian descent, an effect that could be modulated by other environmental factors.