The internet portal began informal discussions with agencies nearly three months ago in a bid to rethink its consumer PR activity (PRWeek, 9 August), although at the time it denied speculation a formal pitch would take place.
Cow's appointment sees three-year incumbent Fleishman-Hillard lose the consumer aspect of its remit, but the agency continues to be retained for corporate and crisis work.
F-H, which declined to repitch for the brief, has handled consumer programmes for Yahoo! since 2000.
Cow's appointment coincides with the news that Firefly Communications account manager Jon Wayth is joining Yahoo! in mid-November as UK consumer PR manager.
Wayth, who leaves Firefly after two-and-a-half years, replaces James Gould, who has left to join auction house Christie's as an art dealer.
Cow reports into Wayth and Yahoo! head of PR Georga Douglas.
A Yahoo! spokesman said it wanted to bring on board a specialist consumer shop rather than a generalist consultancy.
Cow co-founder Dirk Singer leads a team of five on the brief. He said part of its work will be to position Yahoo! as a media business rather than an internet business: 'We want people to see it more as a broader media organisation.'
Cow will also seek to highlight the various content areas within the portal - such as business, sport, shopping and travel.
'Our campaign is all about getting the core audience of 16 to 35-year-old men to buy into Yahoo! as a brand,' said Singer.
The agency will co-ordinate a retainer brief comprising brand building work, product PR, word-of-mouth and guerrilla marketing techniques.