BBH is launching the fund in partnership with Spark Venture Management, a venture capital company whose previous investments have included lastminute.com and notonthehighstreet.com.
BBH's brand invention unit, Zag, is providing an initial "cornerstone" investment to start the fund, which is aiming to raise £10 million to provide capital for start-ups. It will also offer the skills and expertise to help new businesses establish themselves and grow.
Zag, which was launched in 2006 and floated on AIM last year, has a market value of £10 million. The unit's launches include Ila, the rape alarm brand, and Bo Peep, an accessories brand.
Neil Munn, the Zag chief executive, said: "The Black Sheep Fund is a really exciting new way for us to identify and invest in brand-based start-ups. Our approach to deal flow and due diligence is very complementary to that provided by Spark. Our ambition is that The Black Sheep Fund becomes a competitive source of funding in the market place."
Tom Teichman, the chairman of Spark, said: "Now we will be harvesting and harnessing the brilliant start-up ideas in a systematic way."