Umaga, who has turned out for Counties-Manukau in the national provincial championships this season, said he was looking forward to his new role.
"I'm pleased that I'm still able to contribute both on and off the field," he said in a statement. "Playing for the Chiefs was a good fit, as I'll be able to continue to work with some of the key Counties-Manukau players, like Fritz Lee and Tim Nanai Williams."
Chiefs head coach Ian Foster said Umaga would bring valuable experience to the franchise.
"We've been really impressed with Tana as a player and his impact with Counties on his return from Europe," Foster said. "We believe he will be a valuable asset to the team as he brings a high level of experience, knowledge and leadership."
Umaga has been involved with Super Rugby from its inception in 1996 and has played over 300 first class games.
