The 48-year-old singer supported Lopez at her All I Have concert at Zappos Theater at Planet Hollywood prior to reports she will have her own residency at the casino.
Stefani shared a photo Sunday on Instagram of herself backstage with Lopez. The "Used to Love You" singer wore a black dress with embellished tulle sleeves, with Lopez in a white dress with a buckle at the neck.
"@jlo your Vegas show was so good! #eyecandy @inspired #jennyfromtheblock #justagirlfromanaheim gx," she captioned the picture.
"She went to the show last night to check out the venue she'll be playing," a source said. "She was dancing the whole night!"
Stefani released her third studio album, This is What the Truth Feels Like, in March 2016, and followed up with the holiday album You Make It Feel Like Christmas in October. The latter album includes a song with her boyfriend, country star Blake Shelton.