The Voice judge, 36, tells Hollyscoop that he persuaded Clarkson's boyfriend Brandon Blackstock -- who serves as his manager -- to propose. (Clarkson, 30, announced the big news on Dec. 15 on Twitter).
"I told Brandon, man, you need to grow up and figure out that you need to marry this girl," Shelton recalled telling Blackstock.
The country singer, who is married to Miranda Lambert, even bribed Blackstock (the stepson of Reba McEntire) to help seal the deal for Clarkson.
"I am going on the record saying, I will do the music at the wedding or whatever you want to do, I'll be that guy," he said. "I will play for four hours if I have to, cause you need to get your head out of your ass and ask that girl to marry you."
And it worked!
Clarkson, 30, accepted Blackstock's proposal Dec. 14. "I'm engaged!" she tweeted the following day, with a photo of her yellow canary diamond ring. "I wanted y'all to know! Happiest night of my life last night! I am so lucky and am with the greatest man ever."