Financial terms of the deal were not disclosed, although reportedly Orange has paid in the region of £15m for Unanimis, one of the few remaining independently owned online sales houses.
Orange said the combined ad sales network would hand it reach of more than 70% of internet users in the UK.
The sales house employs 68 people, while the online division of Orange is 36-strong, according to Unanimis. The company said the two sales teams would initially remain separate, although they would become more integrated in time.
Unanimis will continue to operate under its brand name and will become part of the Orange Group Advertising Network.
Paul Francois Fournier, executive vice-president, Orange Advertising, said: "With the acquisition of Unanimis, Orange now has strong footholds in two of Europe's three main digital advertising markets - France and the UK - which provide outstanding opportunities to pan-European advertisers and agencies, while enabling us to accelerate our plans for growth from the mobile and online environment."