Brine founded Gryo in 1991 but left his position as chief executive in 2008, after which it merged with US agency HSR.
As well as hiring Brine, RPM is promoting board members, creating a new layer of management, and launching two new teams.
The agency is currently recruiting for staff, after a year in which it extended its work for Cadbury, Sky and Diageo and new client Hasbro.
The two new teams involve planning, concept and digital functions.
Managing partner Hugh Robertson has become group chairman, while client services director Dom Robertson has been promoted to managing director.
A leadership team has also been formed, reporting into Dom Robertson.
Brine's role is his first official post since taking a break from the industry in 2008.
He founded Gyro in 1991 and left as the business had reached a headcount of 700 spread over 16 offices, in the UK and overseas. Richard Glasson replaced him as chief executive.
RPM is "packed with specialists and working with brilliant brands," Brine said. "I've been waiting for the right challenge, and this fits the bill perfectly."
Established in 1993, RPM remains independently owned. Clients include Sky, Diageo, S&N, FIFA & Cadbury.
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