The agency won the account after a competitive pitch against TBWA, Ogilvy and HMDG. The pitch was overseen by AAR.
Now will be responsible for improving BT Business' position in the rapidly growing information and communications technology market for small and medium-sized businesses, through advertising and content creation.
The former incumbent, Rainey Kelly Campbell Roalfe/Y&R, won the account in 2006, when the billings on the business were £17m. However, the agency was forced to resign the account after being appointed to the £47m Vodafone business in August.
Nick Ratcliffe, marketing director at BT Business, said: "We're excited to be working with Now to make it clear to UK businesses how BT's unrivalled range of products and services, from superfast broadband to IT Services, can help them succeed. We were impressed by Now's understanding of our business and its strategic vision."