The investment into the UK online business is part of the German-based publisher's ambition to expand its digital business in the UK.
Axel Springer and PubliGrope together already own online marketing services provider Zanox, based in Berlin.
Jens Müffelmann, head of the electronic media division of Axel Springer, said the acquisition allows it to continue its international expansion.
Founded in 2000, Digital Window enables advertisers to market their goods and services through more than 80,000 online publishers.
The company earns a commission for every successful transaction completed via its platform. Last year, annual sales were in excess of €40m. Clients include Dixons Store Group, Nike and Vodafone.
Based in London, the company employs nearly 100 marketing specialists. The two founders, Kevin Brown and Andrew Totten, will hold 49.9% of the company's stock, with Brown remaining as chief executive.
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