The invention relates to a method and a device for joining a planar component (2) to a hollow section (1). According to said method, the component (2) and the hollow section (1) are positioned in relation to one another in a hydroforming die and are subsequently joined with the aid of a fluidic internal high pressure in the hollow section (1). The aim of the invention is to join a planar component (2) and a hollow section (1) in a simple and reliable manner. To achieve this, under the prevailing internal high pressure, the adjacent walls (3,5) of the hollow section (1) and component (2) that are to be joined are impinged by a punch (6) that is integrated into the hydroforming die, in such a way that the wall (3) of the hollow section (1) is perforated. The result of said perforation is that the punched waste (7) remains attached at one point on the border (8) of the hole, increasing in width towards its free end (9), and that a section of the wall (5) of the component (2) is bent in the form of a tongue (10), which has the same shape as the punched waste (7), into the hollow section (1), to engage behind the border (8) of the hole in said hollow section (1).