The No Doubt singer announced the news on Instagram with a photo of her hand holding a microphone and doing the show's signature peace sign.
"It's true! gx @nbcthevoice," Gwen, 44, wrote alongside.
The new mum will be taking a seat on the panel next to veteran judges Adam Levine and Blake Shelton, as well as fellow newcomer Pharrell Williams.
NBC has confirmed that Christina Aguilera, who served as a judge for season one, two, three and five, will return to The Voice for season eight. She is taking time out to welcome her daughter with fiancé Matt Rutler.
Christina has already tweeted her congratulations to her replacement. "So excited to have you join #TheVoice squad @Gwen Stefani," the pregnant star wrote on Tuesday.
"Welcome to our crazy fam! Kisses xoxo #blondesdoitbetter."
The news of Gwen's new role comes two months after she welcomed her third son, Apollo Bowie Flynn Rossdale, with her musician husband Gavin Rossdale.
The couple, who tied the knot in 2002, are also parents to Kingston, who will be eight next month, and five-year-old Zuma.