A device for reinforcing a structural element in a motor vehicle including: a reinforcing element which has a longitudinal axis and which is disposable in a cavity of the structural element, wherein the reinforcing element includes side walls which extend substantially in the direction of the longitudinal axis, wherein a fastening element for the preliminary fixing of the reinforcing element to the structural element is disposed on a first side wall, wherein a side wall that is opposite the first side wall at a location that is opposite the fastening element has a closed surface for connecting to the structural element.