A reinforcement part for a roof side member of a motor vehicle is provided. The reinforcement part is configured in one piece and includes a first cross section, a second cross section, and a third cross section. The first cross section steadily merges into the second cross section and the second cross section steadily merges into the third cross section. The first cross section, the second cross section and the third cross section vary in vehicle longitudinal direction. The first cross section, the second cross section and the third cross section have a steady contour on a circumference of the reinforcement part and are of a closed configuration.