It is only just over a year ago that the South African left International Sports Management, the stable whose clients include Lee Westwood and Darren Clarke, but he said: "IMG has a global network of offices, loads of experienced and dedicated golf staff operating in every aspect of the game, so really is best equipped to help me achieve everything I want to do on and off the course."
Els was being handled by Pros Inc and his team there are moving to IMG with him.
Guy Kinnings, of IMG, commented: "Ernie has had an amazing career to date and his influence on the game is extraordinary.
"We are delighted to be working with Ernie and his team and know that IMG's offices and staff situated all around the world will be excited to have the opportunity to work with one of golf's most inspiring figures."
Els was scheduled to be in Bermuda this week competing at the PGA Grand Slam of Golf, but he pulled out because of an ankle injury.