The "Humble and Kind" singer also shared a more recent shot of him and Hill, this one showing the longtime couple from behind as they walk backstage at the concert. In the cheeky pic, both McGraw and Hill place a hand on their partner's butt.
"Another pic for our anniversary......just for fun :)," he quipped in the caption.
When ET caught up with the couple ahead of their 21st anniversary, McGraw gushed over his "hot wife," while Hill quipped that "the best gift ever" is her husband's impressive physique. Back in June, McGraw told ET why he decided to work show hard to get healthy and stay in shape.
"Here's the truth of it -- I was not taking care of myself. I was leaning on the success that I've had," he explained at the time. "My family is counting on me. I want to see my kids grow up, I want to be able to spend time with my grandkids. I want to take them diving and spearfishing, all those things that I want to do with my grandkids."
"All the people who work really hard for me and count on me to show up -- I wasn't doing that," he continued. "And I thought that if I want to have a long career and I want to be in this position for a long time, it's easy to bail out now. And I don't want to bail out."