The two celebrated by performing their duet "Go Ahead and Break My Heart" at Country Jam in Grand Junction, Colorado. Stefani, 46, surprised concertgoers by hopping onstage at the end of Shelton's two-hour performance.
Stefani retweeted a fan's video of her surprise appearance Saturday night.
The former "No Doubt" frontwoman also posted several childhood photos of Shelton in honor of his birthday.
She captioned one photo on Twitter: "Happy birthday to my favorite person I ever met @blakeshelton gx #superbabe."
Stefani also took to Instagram to share another throwback photo of Shelton, with the caption: "Happy birthday to @blakeshelton literally my favorite person gx."
In addition to the older photos, Stefani posted a current photo of herself blowing a kiss to the birthday boy.
Stefani and Shelton began dating publicly last November.