To investigate the effects of maternal 25-hydroxycholecalciferol (25OHD) supplementation during lactation on nutrient digestibility and milk composition of sows and gut bacterial metabolites and their metabolites in the hindgut of suckling piglets, 24 Large White × Landrace sows were assigned randomly to one of two dietary treatments (Diet ND: 2000 IU vitamin D/kg feed; Diet 25-D: 50 μg 25OHD/kg feed). The experiment began on d 107 of gestation and continued until weaning on d 21 of lactation. Maternal 25OHD supplementation increased ( < 0.05) total litter weight gain during lactation. Milk fat content, immunoglobulin G level on d 21 of lactation and 25OHD concentration on d 7, 14, and 21 of lactation were higher ( < 0.05) in sows fed with 25OHD. Apparent total tract digestibility of dietary calcium was higher ( < 0.05) in 25-D sows than ND sows. With respect to fatty-acid profile, C16:0 and saturated fatty acids in milk were higher ( < 0.05), but C20:4n-6, the ratios of monounsaturated fatty acids to saturated fatty acids and polyunsaturated fatty acids to saturated fatty acids were lower ( < 0.05) in 25-D sows than ND sows. 25OHD supplementation increased the mRNA expressions of acetyl-CoA carboxylase α and fatty-acid synthase in the mammary gland of lactating sows. For gut bacterial metabolites, concentration of butyrate in the caecal digesta was higher ( < 0.05) in piglets suckling 25-D sows than piglets suckling ND sows. In conclusion, 25OHD supplementation in maternal diets changed dietary calcium digestibility, milk composition and milk fatty-acid profile of lactating sows and altered gut bacterial metabolites in the hindgut of suckling piglets.