A device (10) and a method for transverse sealing of a tube (12) of thermoplastic coated packaging material are provided. The tube is formed by sealing of an overlapping joint (14) between two mutually opposing edge sections (26, 28) of a sheet of packaging material. The device comprises first and second opposing jaws (16, 18) having first and second main sealing surfaces (20, 22), respectively. The tube is arranged to be flat-laid and squeezed between these main sealing surfaces to heat the thermoplastic there between and obtain a transverse seal along a main sealing line (24) of the tube. The device is characterized in that it is arranged to penetrate the outer edge section (28) of the overlapping joint along the main sealing line prior to sealing to allow the first main sealing surface (20) to contact the inner edge section (26) of the overlapping joint.