Its $66.50 per share cash offer means Microsoft is paying an 85% premium to aQuantive's Thursday closing price of $35.87.
It is the fourth major acquisition of an internet ad technology services company within a month, following moves by Google, Yahoo and WPP.
Google made the first move in mid-April by spending $3.1bn on DoubleClick, a move which led Microsoft to cry foul about Google's dominant position in online advertising.
Yahoo reacted two weeks later by paying $680m for the 80% of Right Media it did not previously own.
The entrance of marketing services group WPP, which yesterday agreed to buy 24/7 Real Media for $649m, illustrated the interest in the sector. WPP said one of the reasons for the deal was to strengthen its hold in ad-serving technology.
Microsoft said its deal would enable it to strengthen relationships with advertisers, agencies and publishers by enhancing its existing ad platforms and services.
Steve Ballmer, chief executive of Microsoft, said: "The advertising industry is evolving and growing at an incredible pace, moving increasingly toward online and IP-served platforms, which dramatically increases the importance of software for this industry."
The technology part of aQuantive is Atlas, which provides agencies and advertisers with the Atlas Media Console and publishers with the Atlas Publisher platform.
Its DrivePM network matches ads to publisher inventory to maximise return on investment.
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