Under the terms of the agreement, due to be completed in the third quarter, WPP will make an all-cash purchase of 24/7 Real Media at $11.75 per share.
The deal comes after WPP's rival Publicis Groupe bought US digital and direct marketing company Digitas for $1.3bn (£660m) in January and Google acquired Double Click for $3.1bn (£1.57bn) in April.
Chief executive Sir Martin Sorrell said the acquisition would enhance WPP's position in digital media and marketing while strengthening its technology skills.
24/7 Real Media, founded in 1955, is headquartered in New York. It has operations in 12 countries in North America, Europe and Asia-Pacific.
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