Yahoo announced on Monday that it will acquire Right Media, the creator of an online advertising exchange, for $680 million.

The transaction follows Yahoo's purchase of a 20% stake in Right Media in October 2006.

Founded in 2003, New York-based Right Media operates the Right Media Exchange, an online advertising marketplace utilized by more than 20,000 buyers and sellers.

"This acquisition is an important step in our long-term vision to build the industry's leading advertising and publisher ecosystem," said Yahoo chairman and CEO Terry Semel.

"We believe that Yahoo's open approach is a clear differentiator from others in the industry and provides significant benefits to advertisers, publishers and Yahoo itself."

Yahoo's purchase of Right Media comes just weeks after Google announced its intention to buy online ad firm DoubleClick for $3.1 billion.