Stefan Bardega, president of iProspect EMEA, is to add the UK chief executive role to his remit.
It follows the departure of Jack Swayne, who left the agency after just six months to join M/SIX.
Bardega served as UK chief executive for two years in 2017 and took on the EMEA position in March.
He takes on the additional responsibilities starting next month and will report to Matthew Platts, executive director for media and performance at Dentsu Aegis Network UK and Ireland.
Euan Jarvie, chief executive of Dentsu Aegis Network UK and Ireland, said: "Stefan has proven in his previous spell at helm of iProspect UK – and indeed throughout his career – his know-how in harnessing the latest digital techniques to deliver business results for clients.
"He’s a real industry heavyweight and bringing him back to the UK business, alongside his EMEA role, will ensure it remains a leader not just in our wider market but as an example of best practice for the EMEA region."