Andras Vigh, the global boss of Blue 449, is leaving the company.
There is no word about whether its parent, Publicis Media, will replace Vigh, who has worked at the French-owned ad group for more than 20 years and is based in London.
Vigh has run Blue 449 since September 2015, first as global chief executive and then as global brand president after Publicis Media merged the business with Optimedia in 2016.
Phil Georgiadis, global chairman of Blue 449, said: "Andras is stepping down as global brand president and is in discussions with the group, exploring other options."
There is speculation that Publicis Media is considering a wider shake-up at the media agency network.
Georgiadis said: "We are conducting a review and, for the moment, it’s business as usual."
Publicis Media, the world’s second-biggest media buyer after WPP’s Group M, runs four global media agency networks – Zenith, Starcom, Spark Foundry and Blue 449 – as well as digital agencies Performics and Digitas.
Zenith and Starcom are established brands, but Spark Foundry and Blue 449 are newer creations and have less global scale.
Blue 449, which has its roots in London, is strongest in Europe, while Spark Foundry, previously known as Mediavest, is bigger in the US.
Publicis Groupe has 100% control of Blue 449 around the world except in the UK, where it owns 75%.
M&C Saatchi retains 25% of the UK operation after selling a majority stake in the agency, previously known as Walker Media, in 2013. Blue 449's clients include Asda and KFC in the UK.
The Blue 449 name is meant to signify the fusion of creativity and data. It is named after a tone of blue that is one removed on the colour spectrum from a "perfect blue", Blue 450.
Publicis Groupe's chief executive, Arthur Sadoun, has adopted a strategy called "Power of one", which is focused on serving the group's top 30 global clients and handing power to country managers who can bring together creative, media, data and marketing technology.
He has insisted that individual agency brands remain important.
Publicis Groupe’s supervisory board met this week to give Sadoun a new, four-year mandate, following his appointment in Feburary 2017.
"We have accomplished a lot in these last 18 months to be able to bring our clients a model that will make them win in the future," Sadoun said. "There is still much to be done to improve our performance."