The contract, which began last month, was given to Seymour without a competitive pitch as the agency and its new client had previously worked together on a consultancy basis.
Kate Toland, UK marketing lead at Fitness First, said: "We want to kickstart national conversations about how to incorporate fitness into all aspects of your life, with the ultimate aim of bringing health and fitness to more people."
Carie Barkhuizen, founder of Seymour, said: "Fitness First is an exciting win for us, it’s exactly the kind of content rich brand that lends itself so naturally to those fantastic 'water-cooler' stories that get the attention of media.
"Health and fitness is a growth industry with consumer interest at an all time high, so while it’s a highly competitive area, it’s about teasing out the stories that are most relevant to Britain’s population right now."
There are 150 Fitness First gyms across the UK and a further 220 overseas. Its rebrand will involve changing its corporate colour from blue to red, and moving to a more premium market position.
Fitness First also works with Halpern PR, which provides press office and consumer PR functions.