Metallization in pure hydrogen has been proposed to give rise to high-temperature superconductivity at pressures which still lie beyond the reach of contemporary experimental techniques. Hydrogen-dense materials offer an opportunity to study related phenomena at experimentally achievable pressures. Here we report the prediction of high-temperature superconductivity in yttrium hydride (YH3), with a T(c) of 40 K at 17.7 GPa, the lowest reported pressure for hydrogen-dense materials to date. Specifically, we find that the face-centered cubic structure of YH3 exhibits superconductivity of different origins in two pressure regions. The evolution of T(c) with pressure follows the corresponding change of s-d hybridization between H and Y, due to an enhancement of the electron-phonon coupling by a matching of the energy level from Y-H vibrations with the peak of the s electrons from the octahedrally coordinated hydrogen atoms.