24/7 Real Media is up for sale and is expected to fetch more than $600m (£300m). It was founded in 1995 and is currently publicly quoted on the US Nasdaq exchange.
The company provides digital ad management services including targeting, tracking, analytics and search marketing strategy.
It is a rival to DoubleClick, which was acquired by Google two weeks ago for $3.1bn, building fears that Google is becoming too dominant in the internet advertising space.
WPP chief executive Sir Martin Sorrell has revealed he is concerned about the combination of the two companies and how much advertiser data they would share between themselves.
"Google is a short-term friend and a long-term enemy and probably the shorter term just got a little bit shorter and the longer term got a bit closer as a result of the DoubleClick acquisition," he said last week.
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