Elliot will start her new role on 4 January and will work closely with RTPI managing director Sara Drake and director of professional services Sue Percy.
At GOWM, Elliot was responsible for advising on large and controversial applications. In her role as chief executive of the West Midlands Regional Assembly she worked with councils, businesses and the third sector on the regional spatial strategy.
"Getting out into the regions and nations to meet members and engaging with the forthcoming decentralisation and localism bill are my top priorities," she said.
RTPI executive board chairman Richard Summers said: "I am delighted to welcome Elliot as our new chief executive. She brings extensive senior level leadership and planning experience and has an impressive track record of successfully managing complex organisations with high public accountability."