"Last night at 10:28 PM Ada James Akins was born and it was truly the most amazing thing I have ever experienced," Rhett, 27, wrote. "I can't believe that we have two daughters!!"
The "Crash and Burn" singer added that he has a "newfound respect for moms" following his wife's long labor of 36 hours.
"My wife @laur_akins labored for almost 36 hours," he continued. "She is by far the strongest human being I have ever met and I have a newfound respect for moms around the world. We are so excited that Willa Gray has a baby sister! Thank you Lord and everyone for the prayers."
Rhett added the "#outnumbered" hashtag, referencing the couple's recently adopted daughter, 1-year-old Willa Gray.
Akins also posted the same snap to her Instagram account, referencing the 36-hour labor, "Only Jesus got me through that, and she is sooo worth it. She weighs 7 pounds 3 ounces and is so stinking cute I just can't take it!"
The proud mom added that the new family of four are "doing well now, just resting."
"Thank y'all for the prayers -- she is a miracle and God is so good," she continued. "Thomas Rhett and I are so [in] love with her. And Willa Gray is so excited to finally have a baby sister."
In February, the couple announced that they were not only expecting, but they were also in the process of adopting a baby girl from Africa. The GRAMMY nominee later told ET's Sophie Schillaci that he planned to make any and all adjustments necessary to be able to take his family on tour.
"We've already added a crib to our tour bus," he said. "We're trying to make all the right alterations to make sure that our kids can come [on tour]."
The singer added that he made sure to stock up on sleep in anticipation of the couple's new arrival. (Smart move, Rhett!)
"I wish I got a break, but I doubt it's gonna happen," he told Schillaci. "We're just gonna try to integrate them into our lives as much as possible and try to just create as much of a normal lifestyle as possible. I'm getting all the sleep I can right now, because I know that I'm about to not sleep ever."
As for having two girls? Rhett is elated!
"Before we even started wanting to have kids, I wanted girls. I'm just gonna love them so much," he confessed. "I'm nervous about it as well -- I don't want to deal with boyfriends down the road, you know?"
For more on the happy parents, watch the video below.