The agency beat RMG Connect in the final shoot-out after a three-month pitch process. AKQA has been briefed to develop a global digital strategy, including a website, advertising, customer communications and mobile, across 18 territories.
The agency's new division, MS&A, has also been appointed to handle media search and analytics.
Guy Wieynk, the European managing director of AKQA, said: "We are delighted by this global appointment. It is a really exciting brand with huge potential to deliver real value to its members through digital experiences, as well as attract a new universe of customers."
The appointment of a digital agency follows a review of its above-the-line account, which is currently down to the final stages. Fitness First, whose first club opened in 1993, was acquired by the private equity group BC Partners in 2005 for £835 million.