The country star was headlining pal Luke Bryan's four-day 2017 Crash My Playa event in Riviera Maya, Mexico when he was joined for an encore of his Footloose cover song by Stefani.
The No Doubt frontwoman also performed her band's song Hella Good and later showcased photos and video from the show on Snapchat.
Gwen and Blake have been dating since they hit it off while judging U.S. TV talent show The Voice in 2015, and Blake recently admitted he couldn't have imagined he'd fall in love so quickly with such a "good human being" just months after divorcing country star Miranda Lambert in July, 2015.
"It's been just an eye-opener to be with somebody like her...," he told U.S. talk show host Ellen DeGeneres last month (Dec16). "You think Gwen Stefani and No Doubt, (but) she is literally maybe the most normal person that I've ever met in my life.
"It's been good for me to be with somebody that's so grounded and just a good human being with a great heart. And she's hot! Have you seen her?"
Stefani was dealing with the heartbreak of her shattered marriage to Gavin Rossdale when she met Blake, and she recently thanked her boyfriend for "kissing me back to life" as she took home the Icon prize at the Glamour Women of the Year Awards in November (16).
"This year has been mind-blowing," she said in an accompanying video on the Glamour website.