Dey has worked for The Sunday Times for nearly a decade, including two years as its New York correspondent, and is currently business editor. He has also worked for The Sunday Telegraph, Scotland on Sunday and The Scotsman.
He will join Edelman later this year as senior director in the summer, head of special projects for Smithfield. The agency was described as the "last piece of the jigsaw" for Edelman in London when it was acquired in 2015.
Current Smithfield projects include working with CME on its acquisition of Nex, and with Vantiv on its acquisition of Worldpay, as well as retained work with various asset managers.
Dey said: "Business needs a voice. In an environment where even the basic concepts of profit and loss are being questioned by the general public, it is more important than ever for companies to explain effectively what they do and why they do it."
Ed Williams, Edelman UK CEO and vice chairman, Europe said: "Iain is one of the most respected, seasoned and talented business journalists in Britain and understands the complexity of our clients’ current operating environment better than most.
"Alongside Hugh Taggart, John and the Smithfield leadership team, he will be a dynamic driving force in developing our corporate affairs offer."
Bell Pottinger veteran Taggart joined Edelman in November, overseeing more than 150 staff including Smithfield and Edelman UK's corporate reputation and public affairs practices.
Dey's counterpart on the Telegraph, James Quinn, also moved into PR last year.