The agency fought off competition in a three-way pitch shoot-out to handle the first retained social media brief for Microsoft Advertising (MSA).
3 Monkeys' social media team will work with the in-house team at MSA, which oversees advertising on Microsoft's social platforms including Bing, Skype and MSN.
The brief builds on 3 Monkey's work on the company's trade marketing and consumer PR remit, and expands 3 Monkeys' social media team's portfolio.
The appointment follows news that the tech giant is understood to have cut a number of advertising jobs in recent months amid rising costs brought on partly by a shift to cloud software.
The agency team will be led by associate director Sarah Ogden, who will report to Donna Hindson, trade marketing director of Microsoft's A&O division in the UK.
Ogden said that social media was an 'evolving focus for the business'.
She added: 'Microsoft Advertising has already made inroads in social media and this is about taking it to the next level.'