Freshwater CEO Steve Howell said the agency would give the firm consumer expertise and a presence in London, and that the acquisition was part of plans to build 'a larger group'.
Set up by Larissa Trew in 1997, Trew Relations pulled in £390,000 in fees last year and has Foster's Group Wine Estates and Longines on its client list.
It will continue to trade from its Marylebone office under its current name, with Trew as MD.