EasyBus, is part of the easyGroup run by entrepreneur Sir Stelios Haji-Ioannou, which also includes easyJet, easyCar and easyMoney.
The agency was appointed to the account following a pitch against four undisclosed agencies.
The SPA Way has worked with easyHotel, another member of easyGroup, since the start of 2007.
The agency is charged with handling a media relations campaign to extol the environmental virtues of travelling to and from the airport by bus. The project aims to encourage an passengers to try the easyBus service. The company is set to increase its range of routes over the next two years.
The SPA Way associate director Melanie Diamond will head up the account.