A sealing device forming an annular band intended to be associated with a rim for a wheel designed to accept a tire, the rim having a rim bottom extended by two flanges, the device being adapted to be fitted in such a way as to cover at least the rim bottom. The annular band comprises at least one elastomeric material comprising at least one copolymer thermoplastic elastomer containing blocks of polystirene and polyisobutylene, and a polybutene oil. Also disclosed is a mounted assembly having a wheel comprising a rim associated with a sealing device and of a tubeless tire. In addition, a method is disclosed for fitting a sealing device onto a rim.