The pitch, which is being led by Agency Insight, is due to be completed in July, with the first campaign breaking later in the year.
The appointed agency will work independently from Innocent's in-house creative team, which the brand currently uses for its advertising.
The agency will focus on the health benefits of Innocent's products, using a variety of media.
Last year, Innocent, which is celebrating its tenth birthday, abandoned a pitch process for the third time in 15 months, opting instead to develop its own ideas in-house.
Thomas Delabriere, who was recently appointed as the Innocent marketing director from PepsiCo, defended the company's decision to hold another pitch: "I have a lot of experience working with agencies and we're using a pitch expert, so now we can embark on a clear pitch process."