Gwen Stefani surprised the crowd during Blake Shelton's headlining performance at the Watershed Music and Camping Festival on Friday night (Aug. 3), taking the stage for a performance of her own.

Shelton was in the middle of his set at the Gorge Amphitheatre in George, Wash., when Stefani came out for her show-stealing performance. In the video above, Stefani has the massive crowd in the palm of her hand as she gets them to sing her 2005 hit, "Hollaback Girl."

Shelton's set closed out the first night of the 2018 Watershed Festival, which continues Saturday (Aug. 4) and Sunday (Aug. 5) with performances from headliners Brantley Gilbert and Brad Paisley, as well as all-star daily lineups.

We Have Questions About Gwen Stefani's Birthday Gift to Blake Shelton

Stefani likes to surprise Shelton's fans when she gets the chance; she joined him onstage at Country Jam in 2016 to celebrate his birthday by singing a duet on "Go Ahead and Break My Heart," and they've graced the fans with other joint performances, too.

Most recently, Stefani has been headlining a residency in Las Vegas. Her Gwen Stefani: Just a Girl show opened at the Zappos Theater at Planet Hollywood Resort & Casino on June 27, and Shelton was on hand to cheer her on, as well as sending flowers backstage.

Prior to that, she took some time to unwind with Shelton at his tropical-themed lakefront estate in the tiny town of Tishomingo, Okla. She shared several photos from that fun getaway.

See Inside Blake Shelton's Tropical Estate

WOW! Gwen Stefani Has the Craziest Celebrity Mansion EVER!

More From KIX 105.7