MediaCom is set to scoop Adidas's $300m (£229m) global media account after a competitive review.
Dentsu Aegis Network’s Carat has held the account in many markets, including the US and UK, in a relationship going back two decades.
Publicis Media’s Starcom has handled Adidas’ media-buying in the Middle East.
Adidas, which used MediaSense to manage the pitch process, has contacted agencies about its decision.
The sportswear giant did not immediately respond to a request for comment. MediaCom declined to comment and referred enquiries to Adidas.
The result comes midway through the Fifa 2018 World Cup – a key event for the German-owned sports brand, which has been an official sponsor of the tournament for 20 years and sponsors a string of teams, including Argentina, Belgium, Colombia and Germany.
Adidas hiked its annual marketing spend by 13% to €2.73 billion last year, according to its 2017 annual report.
"Marketing expenditure is one of our largest operating expenses and at the same time one of the most important mechanisms for driving brand desirability and top-line growth sustainably," the owner of the Adidas and Reebok brands said.
"Therefore, we are committed to improving the efficiency of our marketing investments. This includes concentrating our communication efforts on key global brand initiatives and focusing our promotion spend on well-selected partnerships with top events, leagues, clubs, federations, athletes and artists.
"We also aim to increase operational efficiency by tightly managing operating overhead expenses."
The subsidiary Adidas brand currently spends almost half of its marketing investments on promotional partnerships including on teams such as Manchester United, Real Madrid and the New Zealand All Blacks, as well as individual sports and entertainment stars such as Lionel Messi, Gareth Bale, Kanye West and Stella McCartney.
The other half is spent on brand marketing activities such as digital, advertising, point-of-sale and grassroots activations.
Adidas said it plans to reduce the ratio of marketing investments spent on these promotional partnerships to less than 45% by 2020.
MediaCom’s win comes despite uncertainty over the future of parent company WPP, which parted ways with Sir Martin Sorrell in April and has not yet found a replacement for the chief executive.
MediaCom decided not to defend another client, Revlon, which awarded its estimated $450m media business to Initiative earlier this week.