Rival Titan had previously handled sales for the Wales and West of England-based contractor, but, under a new agreement, JCDecaux will take over responsibility for selling the 200-strong portfolio from the end of this year, adding further sites to the remit in 2008.
The deal means JCDecaux now has a stronger proposition with its large format offering in areas such as Bristol and Cardiff, as well as Newport, Bridgend and Yeovil.
Mike Parkin, managing director of Harlech Outdoor, said his company would be working with JCDecaux to provide "new opportunities for advertisers on these and other high-profile sites".
He added: "We see this agreement as a sign of JCDecaux's continued investment in roadside inventory."