Patrick Affleck, chief executive of Dentsu X, is set to join Havas Media Group in the same role.
Joining in the summer, he will replace Matt Adams, who quit Havas in January to become global managing director of Brainlabs. Affleck will be responsible for leading and growing the 500-strong Havas Media Group in the UK.
Affleck has worked at Dentsu Aegis since 2008, the majority of that time at Vizeum. He joined Vizeum as managing partner from Profero, later becoming global head of innovation and president of Vizeum in the US.
Peter Mears, global chief executive of Havas Media Group, said: "Patrick is a first-class leader, with a proven track record in leading and developing multidisciplinary media businesses.
"I’m delighted to welcome him to Havas Media Group and look forward to seeing how he helps us to continue to deliver more meaningful media experiences for our clients in the UK."
Affleck is the second figure this month to make the jump from Dentsu Aegis to Havas, following Jackie Lyons, planning partner at Carat, who is joining Havas Media in the new position of head of media experience.
Other senior figures to have left Dentsu Aegis in recent months include executive director for media and performance Matthew Platts, who was replaced by former Guardian News & Media chief revenue officer Hamish Nicklin.