An apparatus for capping a container (10) comprises: - a conveying device for conveying the container (10) along a path; - an applying head (23) for applying a lid (1) onto a neck (9) included in an opening device of the container (10); - a welding head (43) arranged downstream of the applying head (23), the welding head (43) comprising a welding element (51) for welding the lid (1) to the opening device.