The country singer revealed the exciting news on Instagram on Tuesday night.
Rhett also revealed that they named the baby Lennon.
"Lennon Love Akins was born at 8:30 AM on February 10th," said the caption. "It was such a joy to watch this little angel be brought into the world. My wife is just incredible through the entire birth."
He added: "Watching our kids meet Lennon for the first time was probably the sweetest thing I’ve ever seen in my whole life! Ada James and Willa Gray are going to be the best big sisters on the planet. The Lord is so good and I’m so grateful for my beautiful growing family!! Y’all pray for us as we switch from man on man defense to zone."
The couple – who are already parents to daughters Willa Gray, 4, and Ada James, 2 – announced in July 2019 that they were expecting their third bundle of joy.
"Excited to share that I will now be paying for 3 weddings😂 I feel so blessed to say that our third child is on the way and it’s a girl!! I love you so much @laur_akins. WG and AJ, y’all are gonna be the best big sisters! Bring on the crazy," the "Life Changes" singer, 29, said at the time.
Meanwhile, Lauren, 30, wrote: "We decided we didn’t have enough princess dresses around the house so we’re adding another Akins baby girl to the family early next year despite Thomas Rhett’s initial shock (swipe to see video 😂) we are absolutely over the moon for our baby girl! Our girls cannot wait to meet their little sister."
Lauren also admitted that she felt like the family had been "hiding this secret for an ETERNITY.”
Rhett and Lauren have previously opened up about their fertility struggles and decision to adopt Willa from an orphanage in Uganda.
The couple had been struggling to get pregnant and at the same time, they were working Christian charity 147 Million Orphans. When they met Willa, they decided adoption was the right choice for them.
During the adoption process, the Rhett’s received the surprising news that they were expecting. They welcomed Ada in August 2017.
In an interview with his record label, Big Machine, published on Feb. 3, Rhett gushed over the idea his family would soon be expanding once again.
“We’re all really excited,” he shared. “Before I had kids, I never really imagined that I would be this at-peace with two, and one on the way. I thought I’d be like, ‘What in the world is happening?’”
And as he gears up to go on his Center Point Road tour, which is set to begin in May, Rhett’s hectic lifestyle doesn’t seem like it will be slowing down anytime soon.
“Me and Lauren were actually talking the other day about, even though our lives are insanely chaotic, and we never really stopped going and we just keep growing our family, we really do feel, right now, that we’re at a more peaceful spot than we’ve ever been,” he said.
“Maybe we found the calm in the crazy somehow, but we’re excited.”
Fox News’ Sasha Savitsky contributed to this report