Michaels, who originally left the show in 2011, will join returning trainers Bob Harper and Dolvett Quince for the 14th season of the show, NBC announced today.
The network also revealed that the new episodes, which will premiere in January, will feature teenage contestants aged between 13-17 alongside adult contestants.
"As a former overweight teen, I know firsthand how dramatically weight issues can affect every aspect of a child's life," Michaels said.
"Having recently become a mother of two, I am more passionate than ever about helping empower children and families with the information and resources they need to live a healthier life."
The contestants in the new season will be divided into three teams, with Michaels, Harper and Quince each mentoring a group. Each team will include a teenage contestant, but they will not be eligible for elimination and will work at both the ranch and at home.
NBC also promised that "special celebrity guest stars and additional surprises" will be revealed later.
The last season of The Biggest Loser, won by Jeremy Britt, featured famous faces including Michelle Obama and Jessica Simpson.