A sheet-material piercer (1) for a container closure (2) of the push-pull type is of essentially rotationally symmetrical design and can be inserted into the container closure (2) in the direction of the axis of rotation of symmetry (R) such that it can be displaced axially independently of the push-pull part of the container closure (2). The sheet-material piercer (1) is in one piece and has at least one sealing formation (16, 17, 18) which is likewise of rotationally symmetrical design and is arranged between an actuating end (13) and a piercing end (14). The sealing formation (16, 17, 18) is designed for sealing and latching contact with the container closure (2). Also proposed is a container closure (2) for a container (3) with a so-called push-pull closure and such a sheet-material piercer (1).