The agency won the business after a competitive pitch last month.
Hypernaked will be responsible for developing Actimel's approach to engaging consumers online, following its repositioning in January as the 'Little bottle with a lot inside'.
The repositioning included a new TV ad created by Actimel’s advertising agency, Rainey Kelly Campbell Roalfe/Y&R, and was followed in-store earlier this month with revamped packaging.
Joel Kirstein, assistant brand manager at Actimel, said: "The standard of the responses to the brief was very high, but Hypernaked really stood out throughout the pitch process for their insights, strategic thinking and ability to translate this into excellent creative work."
Brad Fairhead, the managing director at Hypernaked, said: "Actimel combines an internationally recognised brand, big-business ambitions and a category going through significant change.
"It's a combination we love to work with, so we can't wait to get stuck in and make a difference."