A connection assembly for conducting a medical liquid, comprising a connection piece, an attachment part attached to the connection piece； an opening into which the connection piece engages； and a seal element for sealing a passage between the connection piece and the attachment part. The seal element is held between the attachment part and the connection piece and has a sealing head, which closes the opening of the attachment part against the passage of liquid when the attachment part is attached to the connection piece, and a body, which adjoins the sealing head and which can be arranged in the opening at a radial distance to the attachment part.
