The placenta is a unique bond between the mother and the fetus during pregnancy, and a proper placental angiogenesis is vital for fetal development. HS is an endogenous stimulator of angiogenesis that is mainly produced by the methionine transsulfurationpathway. The goal of this study was to evaluate the effect of gestational dietary methionine on maternal and placental HS production in sows. Multiparous sows (Large×White; third parity; = 65) were randomly allocated into five groups, with feed diets comprisingstandardized ileal digestible methionine/lysine (Met/Lys) ratios of 0.27 (nutrient requirements of swine (NRC); 2012 level), 0.32, 0.37, 0.42, and 0.47, respectively. The litter size and weight at birth were measured and recorded. Maternal blood samples were obtained at embryonic day (E) E40 d, E90 d, and E114 d of gestation. The placental samples were collected at parturition. The results showed that maternal plasma HS concentration was not affected at E40 d. However, the maternal plasma HS concentration changed quadratically with the dietary Met/Lys ratio at E90 d ( < 0.01) and E114 d ( = 0.03). The maximum maternal plasma HS concentration was at the dietary Met/Lys ratio of 0.37. Meanwhile, maternal plasma HS concentration was positively correlated with piglets born alive ( < 0.01) and litter weight ( < 0.01). Consistent with the maternal plasma, the placental HS concentration also changed quadratically with the dietary Met/Lys ratio ( = 0.03); the Met/Lys ratio of 0.37 showed the maximum HS concentration. In conclusion, our findings revealed that the gestational dietary Met/Lys ratio could affect maternal and placental HS concentrations, which may be an important molecular mechanism affecting placental angiogenesis and piglet development.