Incumbent Universal McCann will continue to manage Microsoft's media planning and buying outside of North America.
Starcom Mediavest will handle buying in the US and Canada, while Universal McCann will handle some 35 markets outside of North America.
Microsoft spends about 60 per cent of its ad budget in North America and about 40 per cent overseas.
Previously Universal McCann handled all Microsoft's ad strategy, planning and buying, except in Brazil and China.
It remains unclear how Starcom's appointment will effect the business in Asian markets.