NZPA 15.Mar.2010Getty Images
Counties Manukau chief executive Phil McConnell said Umaga, who has been coaching at Toulon in France since 2007, was keen to return home.
"The board took action as soon as we became aware that Tana was looking to return home to further his coaching career and be closer to his family," he said.
McConnell said the 74-cap veteran still had plenty to contribute, both as a player and as an assistant coach.
"Tana Umaga is a perfect match for Counties Manakau. His mana, experience, professionalism and dedication are precisely what we need to take our team to the next level."
Counties Manukau coach Milton Haig said part of Umaga's role would be to assist with the coaching and development of the young players.
"We have a group of young and very talented players that we are developing, and having someone with Tana's outstanding attributes as a player and coach will be invaluable for all those involved."
Umaga said he was pleased to be returning to New Zealand.
"My family and I are excited to be returning home, and I am looking forward to the challenge that lies ahead."
Umaga said he had been keeping fit and was looking forward to "putting the boots on again".