A high pressure pump for delivering a fluid includes a pump housing with a longitudinal axis and a housing cavity, a piston partly arranged in the housing cavity and axially moveable within the housing cavity and having an axial end directed opposite a driving side of the high pressure pump, and a sealing unit having a sealing body arranged radially outside of the piston such that a first part of the housing cavity is sealed fluid-tight against a second part of the housing cavity. The sealing body includes a neck with an inner diameter smaller than a first diameter of a part of the piston arranged between the sealing body neck and the axial end of the piston. The piston has a second diameter in an axial area in which the sealing body neck is arranged, which second diameter is smaller than the first diameter.