PR Week revealed in March that Grayling was frontrunner for the comms contract.
The three-year deal covers 38 countries and begins today.
Grayling will manage communications using a hub and spoke model with central hubs for Europe and Asia-Pacific, located in Grayling's London and Hong Kong offices.
Michael Murphy, global CEO, Grayling, commented: 'British Airways is an outstanding addition to our client roster, particularly at this fascinating time for its business and its brand.'
James Acheson-Gray, managing director international, led the pitch and will oversee the programme with the Asia hub led by Bruce Shu, managing director Asia.