The adsorption of water molecules on tricalcium silicate (C3S), which influences the initial hydration of C3S, is still unclear at the atomistic level. In the present paper, density functional theory is employed to depict the adsorption of a single water molecule on seven low-index M-3-C3S surfaces. The calculations show that both molecular and dissociative adsorption can occur on the C3S surfaces and that the latter mode is preferential. All of the ionic O atoms on the C3S surfaces can adsorb the H atoms from dissociated water molecules, while only two-coordinated covalent O atoms on the surfaces can form O-H chemical bonds. The electronic structures of the ionic and two-coordinated covalent O atoms in the first atomic layer of the C3S surfaces show similar charge density localization of the valence band maximum (VBM), which can describe the variations in the reactivity of the ionic O atoms in the bulk or exposed on the surface slab. The partial density of states (PDOS) analysis shows that the formation of new Ca-O bonds is mainly due to the overlap of O-2s and Ca-3p orbitals and O-2p and Ca-3d orbitals. Furthermore, the position of the OH group generated from the dissociated water molecule is found to significantly affect the adsorption energy. The general order of the adsorption energy in terms of the position of the OH group is E-hollow > E-bridge > E-top. The findings in this study provide additional support for the fundamental understanding of C3S hydration.