The appointment is understood to have followed a final two-way pitch which saw it win out against incumbent OgilvyOne.
As a result, Sapient will take over running the airline's digital platforms including the main website BA.com and its digital communications including social media.
Nigel Vaz, senior vice president and European managing director of SapientNitro, said:
“The appointment of SapientNitro as British Airways’ agency is a game-changer all round.
Through a forward-looking approach to the use of multiple digital platforms and creative technology, British Airways has built renewed strength around its brand.
"We are proud to join them on the next leg of that journey and, through our passion to break boundaries at the intersection of technology and story, to give the British Airways brand an ever-greater relevance in the lives of connected consumers.”
Sara Dunham, head of marketing, retail and direct for British Airways, said:
"SapientNitro offer us great ideas, good insights, the ability to apply strategy practically, a strong reputation in the industry and a good fit with the British Airways brand.
We're really looking forward to working with them.”
Last year Sapient Nitro reached the final two in pitching for the British Airways advertising account, ultimately losing to incumbent BBH.