The business, which is currently handled by the media specialist The Piper Agency, also includes the company's trains division. The AAR has drawn up a shortlist of five agencies.
The move comes almost two months after National Express moved its creative account from the incumbent, Inferno, to Hooper Galton following a competitive pitch. That pitch was also overseen by the AAR.
National Express operates an extensive coach network in the UK and Europe. It also runs intercity, commuter and rural passenger train services, as well as urban bus services in Birmingham and across the West Midlands. It is trying to shake off out-of-date perceptions of the brand and is trialling onboard Wi-Fi services and TV viewing systems in its fleet of coaches.
The company has also faced competition from new entrants such as easyGroup's easyBus and Stagecoach's low-fare MegaBus.
The travel specialist appointed its first online planning and buying agency, Media Contacts, last year.