The FTSE 250 firm previously used Weber Shandwick Financial and chose Citigate after a competitive pitch.
The agency will be charged with strengthening Go-Ahead's reputation among financial audiences and key stakeholders as one of the UK's leading transport providers.
Citigate will report directly to group chief executive Keith Ludeman. The account will be led by executive director Michael Berkeley and Chris Barrie, director in Citigate's financial practice.
Ludeman said Citigate was brought on board because of its ‘strategic capabilities, specialist skills and creativity'.
GoAhead runs bus, rail and aviation services and is behind rail franchises Southern, South Eastern and London Midland. It employs more than 27,500 people and provides about 920 million passenger journeys each year.
Customers include the Department for Transport, Transport for London, local authorities, British Airports Authority and major airlines.