Stefani, 48, said on Twitter that it was "incredible" to sing "You Make It Feel Like Christmas" with Shelton on her NBC holiday show of the same name.
"Yay!! It was incredible to write with @BlakeShelton & share the stage for a song that is so joyful & Christmas-y! #YouMakeItFeelLikeChristmas #LuckyGirl," she captioned a gif of the performance.
Stefani hosted the special, which also featured Chelsea Handler, Ken Jeong, Seth McFarlane and NeYo. "You Make It Feel Like Christmas" appears on her holiday album of the same name, which debuted in October.
Stefani said in an interview with On Air with Ryan Seacrest earlier in the day that she will spend Christmas with Shelton and her three sons. The star shares 11-year-old Kingston, 9-year-old Zuma and 3-year-old Apollo with ex-husband Gavin Rossdale.
"I have the kids [this year]," she said. "We were gonna go to Oklahoma first and then come here and go back and forth and now it looks like we're going to be here [in L.A.]."
"We go big," the singer added. "Christmas Eve is Christmas Mass. Then we go to my brother's house and we do Christmas carols."
Stefani and Shelton confirmed their relationship in November 2015 following their respective divorces from Rossdale and Miranda Lambert. Stefani told The Tonight Show in November that spending time at Shelton's ranch inspired songs on her Christmas album.