Aegis will pay an initial £16.7m with a further £9.4m deferred and payable over two years, subject to performance-related targets.
IProspect will become part of Aegis Group's digital media network Isobar, which has made a number of acquisitions over in the past six months, including Korean digital agency Agency W and Australian digital agency One Digital.
Founded in 1996 and headquartered in Watertown, Massachusetts,
iProspect is one of the pioneers in search engine marketing.
Earlier this year, it was named among the top 50 fastest-growing companies by Inc. magazine in the US.
Doug Flynn, CEO of Aegis, said: "Digital media will become the dominant delivery mechanism for media and advertising. Understanding the ways in which the internet can be used to effectively develop consumer interest and drive sales is the holy grail of marketers. We are very impressed with the vision of iProspect and are very keen to develop this part of the business into a true international capability."
Isobar launched last month to integrate all Aegis's digital businesses. The Isobar brand is designed to provide a clearer, more integrated model for clients. It incorporates digital services from 25 offices in 19 countries.
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