The Seattle-based software giant has muscled in on a previously agreed $426m deal for private equity firm Quadrangle to buy Greenfield.
Microsoft said it would integrate Ciao's technology platform, online community and retailer relationships within its Live Search platform.
Ciao was founded in Munich in 1999 and was acquired by US-based Greenfield in 2005. It operates in the UK, Germany, France, Italy, Spain, the Netherlands and Sweden, and is the most popular shopping portal across Europe with 26.5m users per month according to ComScore.
The deal is expected to go through in the fourth quarter, after which Ciao will report to Rajat Taneja, general manager for worldwide search at Microsoft.
John Mangelaars, vice-president of consumer and online at Microsoft Europe, Middle East and Africa, said: "Ciao's success has been led by a team of talented people who took a unique combination of intuitive technology and the insight that comes out of their passionate consumer community to become one of Europe's leading shopping comparison sites.
"This makes the company a fantastic asset to the future of our search offer. Integrating Ciao's capabilities into Live Search will provide a strong launchpad for our commercial search offer in Europe and enhance our e-commerce offering on MSN."
The related sale of Greenfield Online's survey business is also expected to go through in the fourth quarter. No information as to the identity of the buyer Microsoft has found has been released, apart from the disclosure that it is a financial rather than a trade buyer.
This article was first published on brandrepublic.com