The company previously worked with Mother, but that relationship came to an end in autumn last year.
BBH will now work on a campaign for the brand after a process that was overseen by Claire Dobbie, the brand's marketing director.
The retailer does not operate on a retained agency basis, opting instead to assign shops on a project-by-project basis.
ASOS also works with Independents United on its digital work and the media agency Carat. Last year, James Hart, the ASOS e-commerce director, used Twitter to ask for digital agency recommendations.
Recent work for the brand included a series of idents by Mother promoting its sponsorship Britain's Next Top Model for Living TV.
The online retailer, which is estimated to have 5.2 million unique visitors a month, is one of the fastest-growing internet fashion outlets.