The 42-year-old country star marked the occasion Wednesday by dedicating a sweet post to the singer on Instagram.
Shelton shared a sexy photo of Stefani wearing a cropped jacket and yellow shorts. He couldn't help but gush about the former Voice coach in the caption.
"Somehow it feels more like MY birthday everyday.. Happy Birthday @gwenstefani!!! I love you!!!" the star wrote.
Stefani said in an Instagram post in September that she is "so grateful" for Shelton. She posted a video of the country star serenading her with his song "Turnin' Me On," which was inspired by their relationship.
"I'm so grateful," Stefani wrote, adding a heart emoji.
Shelton and Stefani confirmed their relationship in November 2015 following their respective splits from Miranda Lambert and Gavin Rossdale. Shelton said in an interview with Today in June that he feels closer to Stefani every day.
"Now here we are, going on three years later and every day that goes by, it just feels like a stronger bond between the two of us," the star said.
"It constantly feels like it's going to the next level. That's the only way I think either of us could probably describe it," he added.
Stefani announced plans last week to release a deluxe version of her holiday album, You Make It Feel Like Christmas, on Oct. 26. The album includes a duet of the same name featuring Shelton.