Now won the account following a pitch process that kicked-off last October.
Its appointment follows the selection of MediaCom to handle Nuffield's media planning and buying last November.
Beta had previously worked with Nuffield on its advertising activity, having won the integrated business in October 2011.
Now’s work will cover all three elements of Nuffield Health’s operation – hospitals, fitness and wellbeing, and corporate.
Nuffield operates 31 hospitals and 65 fitness centres across the country and is also the UK’s largest corporate fitness provider, with more than 100,000 members.
Mark Lund, a managing partner at Now, said: "Nuffield has a wonderfully compelling vision for improving the UK’s health and a real appetite for growth. We’re delighted to be working with them."
Nuffield Health is a not-for-profit social enterprise and reinvests any profits into patient and customer care.