Initiative won the business in a pitch against MediaCom, OMD UK and AMS, which had held the business for more than a decade.
The agency will work across all media, focusing on TV and press. Its brief is to help the company strengthen its leadership in the cooked meats sector and support brand extensions such as pasties.
Bernard Matthews is a family owned company and handles its creative in-house.
The company was launched in 1950. Its eponymous founder found fame in the early 80s when he coined the "bootiful" catchphrase to support his Turkey Breast Roast product.
Andy Watts, the Bernard Matthews marketing director, said: "We are delighted with Initiative. The strength of the team, both in terms of strategic input and a real desire to provide great value, was highly evident."