The unit, which has signed up the BMB clients Pretty Polly and Pussy Drinks, will be led by Sam Drake, who joins from Starcom MediaVest Group, where he was a strategy director.
The new offering, which will be based in BMB's London office, will target medium-sized clients who "demand joined-up creative and media thinking".
Goodstuff will co-own and support the venture with its strategic input and the media buying scale provided by its minority investor, Omnicom Media Group.
Andrew McGuinness, a founding partner of BMB, said: "Clients increasingly see a competitive edge in ensuring their creative and media thinking is joined up.
"While the biggest clients have the resources to ensure this happens, others are left refereeing between two different agencies. BMB Media makes getting joined-up thinking quicker, easier and more affordable."
Andrew Stephens, a co-founder of Goodstuff, said: "We've worked closely with BMB for more than six years. The time was perfect to more formally bring our companies together."
Separately, BMB has hired Alex Brown, Karmarama's head of account management, to the newly created role of head of talent.