New investors AOL Ventures, Mitsui & Co. Global Investment and Presidio Ventures, and previous backers Accel Partners, Index Ventures and DAG Ventures also participated.
Los Angeles-based OpenX operates a platform that combines an ad server and an ad exchange.
Customers include Groupon, Excite Japan and Business Insider.
The company will use the funds to drive international expansion and further adoption of its OpenX Enterprise platform.