The 46-year-old singer will return to the NBC reality competition as Shelton's team advisor after serving as a coach in Seasons 7 and 9.
Stefani and the 39-year-old country star confirmed the news Wednesday on Access Hollywood in their first interview as a couple. Shelton couldn't help but gush about his girlfriend, saying it was his idea for Stefani to serve as a mentor.
"You start thinking like, 'Who are the ultimate artists out there that you can get to mentor your team?'" he said. "You start thinking -- you do -- 'Wait a minute! This is perfect. And I actually know her. It'll be easy to call her.'"
"We know we collaborate well together as coaches, so of course I would have her as my mentor," he added. "And she's so smart and talented."
Stefani joined The Voice as a temporary replacement for singer Christina Aguilera, who will return as a coach in Season 10. Patti LaBelle will serve as Aguilera's advisor, with Tori Kelly for Adam Levine and Diddy for Pharrell Williams.
Stefani and Shelton split from their respective spouses, Gavin Rossdale and Miranda Lambert, in 2015, and confirmed their relationship in November. The country star showed his support for Stefani's forthcoming live music video Wednesday, tweeting, "Awesome!!!!!" in response to the news.
Shelton has coached four contestants to victory since The Voice's premiere in 2011. The country star last released Bringing Back the Sunshine in 2014, while Stefani will debut her third solo album, This is What it Feels Like, in March.