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, reported Contactmusic.
Gwen and Blake have been dating since they hit it off while judging US TV talent show "The Voice" in 2015.