The invention relates to a device for clamping at least two components 17, 18, in particular components 17, 18 for aircraft, in order to join the components 17, 18 by means of a weld seam 22 formed between two component edges 20, 21 via friction stir welding, wherein the components 17, 18 each are accommodated by several supporting elements 6-16, and a welding backup plate 3 is arranged under the weld seam 22. According to the invention, both sides of the welding backup plate 3 each have at least one mounting device 4, 5 that can be exposed to a vacuum for securing the components 17, 18. The mounting devices 4, 5 that can be placed under a vacuum enable the rapid and easy clamping of components 17, 18 for purposes of joining via the known friction stir welding process. Because fewer parts are exchanged, in particular the mounting devices 4, 5 and/or the welding backup plate 3, the device according to the invention can be easily and flexibly adapted to a plurality of components 17, 18 with varying component geometries and/or component dimensions. The very same device can be used to clamp a plurality of component types, in particular components 17, 18 with single or double curves for the friction stir welding process. Use of the device is here not limited to raw components, e.g., blanks or the like. Rather, complex, prefabricated single components, e.g., entire fuselage shells or partial shells, can be clamped and welded. In addition, the device according to the invention also enables higher rates of advance during friction stir welding, because heat dissipation from the weld formation area 19 can be minimized via the use of poorly heat-conductive materials for the mounting devices 4, 5 and/or the substructure 2.