Shelton opened the iconic American program with "It's All About Tonight," then came back later to join buddy Trace Adkins on their duet "Hillbilly Bone."
"I don't have a whole lot of friends in this business because I tell people what I think," said Adkins, who presented his pal with the honor. "I tell Blake what I think and he don't care. He still talks to me."
Added Adkins: "It means the world to me to say Blake Shelton, you're the newest member of the Grand Ole Opry!"
An emotional Shelton – who put on a necktie for the special occasion – held the bronze-and-oak Opry Member Award and struggled to express how profound the honor was to him.
"This is the reason you come to Nashville if you're me. This is what I wanted from the beginning, to make a mark," he said. "I get the same feeling walking into the Opry House as I do when I see one of my heroes. The Grand Ole Opry is an artist, and I am proud to be one of its songs."
Shelton closed the show with his ballad "Home." His new greatest hits album, Loaded: The Best of Blake Shelton, will be released Nov. 9.