A fuel injection system, and a structure for mounting a fuel injection valve to an internal combustion engine, that includes: a combustion chamber formed at an end surface of a piston that reciprocates in a cylinder; an intake port through which intake air drawn into the combustion chamber flows; and an intake valve that opens and closes a connection between the intake port and the combustion chamber, the fuel injection valve being placed to inject fuel into intake air in the intake port, wherein an end portion of the fuel injection valve located on a combustion chamber side thereof is placed at a location that overlaps with an imaginary plane perpendicular to a center axis of the cylinder and extends along a portion of a wall surface of the intake port where the intake valve protrudes, or is projected out from the imaginary plane toward the combustion chamber.