The agency has taken the business from incumbent Peppercom, after fighting off ten other agencies in a competitive pitch.
The account, believed to be worth a six-figure sum annually, will see Diffusion Mobile tackling an integrated media strategy to target global operators and analyst communities using online, trade and national media.
Diffusion Mobile has also been charged with raising awareness of the brand and managing its profile in the run-up to Mobile World Congress in February 2011.
The campaign will focus on themes relating to consumer response rates and ROI from mobile marketing.
Upstream head of marketing Alexandra Chong said: 'Mobile marketing is a young but crowded market, and to achieve penetration it is vital to engage multiple audiences with very clearly defined messages.
'Diffusion Mobile was selected on the basis of its experience of executing successful campaigns in the mobile and marketing industries.'
Upstream has delivered marketing campaigns for major brands including The Coca-Cola Company, Nestle, Unilever and BSkyB.
The firm claims to have a reach of more than 500 million consumers in 40 countries worldwide through its mobile marketing platform.
Diffusion Mobile was established by Diffusion last year and counts SellMyMobile.com and Volantis among its clients.