"Happy Birthday @gwenstefani!!! Why don't you go ahead and NEVER break my heart!!!!" he wrote, making a play on their duet "Go Ahead and Break My Heart."
Shelton's post was favorited over 17,000 times on Twitter. Stefani responded with an equally emotional message.
"@blakeshelton thanku for saving my life gx," she tweeted, adding heart emojis.
The couple and their families have grown close since, with Shelton hosting Stefani, her three sons and extended family at the opening of his restaurant Ole Red in Tishomingo, Okla., last week.
"It's been a lot of fun for me to be the one to get to expose them to this -- out in the country," the country star told Entertainment Tonight. "They love it so much, her entire family."
"And when I say her family, I mean all of them. I'm talking about the kids, the mom, the dad, the brother," he added. "There's times when we've had holiday gatherings, and I'm talking about 30-plus family members from her side, from my side."
She will release the holiday album You Make It Feel Like Christmas on Friday, which includes a duet of the same name with Shelton.