Phil Edmonds, managing partner at Oystercatchers, is leaving the intermediary after two years in charge.
His departure follows that of Peter Cowie and Angus Crowther last year. Edmonds had been running the organisation since then. His last day at Oystercatchers will be Friday (15 November).
An Oystercatchers spokesman said details of Edmonds' successor would be announced shortly.
Suki Thompson, chair of Oystercatchers and executive director of Xeim, said: "Phil has brought innovation and leadership to Oystercatchers as it transitioned into Xeim. While we are sad to see him leave, we would like to thank Phil for his significant contribution to the business and wish him all the best in his next adventure."
Edmonds joined Oystercatchers from Adstream where he was general manger Europe.
He added: "After two years building Oystercatchers, and gaining great insights and experience in business transformation and pitching, the time is right to leave and take on a new challenge. I have been privileged to work with some of the most dynamic brands in the UK, including Axa, F1, Marks & Spencer, Samsung and Starling Bank. I wish the business all the best in the future."