It is the first time the charity has brought in support as it kicks off a major campaign to celebrate its 50th anniversary.
Forster beat off two other agencies to the 12-month account, which will see it handling regional press.
The charity wants the public to see that it is increasingly sending out experienced professionals to developing areas to share skills in health, education and business.
The account will be headed up by former Shelter comms director Ben Jackson, who joined the agency last year. He is also on the board of the Fairtrade Foundation.