An opening device for a flexible package having an opening portion defined by two side walls and a width, the opening device intended to be disposed in the opening portion of the package adjacent to an opening, formed upon initial opening of the package, for opening and closing thereof. The opening device comprises a thin-walled first body with a contact surface and a first application surface opposite thereto for allowing application of the first body in the opening portion to a first of the two side walls. The first body is maneuverable, by folding about a first folding axis, between a basic position and a sealing position. In the sealing position, the contact surface has sub portions directly facing each other. A locking member is designed for releasable locking of the body in the sealing position.