Openads said it is used by more than 20,000 publishers--more than all its competitors combined.
The move comes in the midst of increasing industry speculation that Google plans to make its ad servers free. The search giant's DoubleClick acquisition would give it that firm's ad server in addition to AdSense.
Index Ventures has previously invested in such companies as Skype and MySQL.
With the new investment, Bernard Dalle and Saul Klein from Index Ventures, and Gerard Lopez from Mangrove Capital, will join Damon Reeve from Unanimis on the Openads board of directors.