We have developed Ag2O-catalyzed allylation reactions of aldehydes with allylsilanes in water, providing homoallylic alcohols in high yields with high anti-selectivities. It was found that the reactions proceeded not in the water solution but on the surface of Ag2O. We could reduce the amount of the catalyst to less than 0.01 mol%. Moreover, an asymmetric allylation reaction was performed in water with high yield and moderate enantioselectivity by using a chiral phosphine ligand.