Jason made the announcement with a photo of the couple's son, Memphis, wearing a onesie that read "Big Brother."
"Sup everybody....... guess who is gonna be a BIG brother!" the singer wrote. "#thisdude #aldeanpartyof6."
Brittany posted the same photo on her own account, writing, "Here we go again!"
This will be the second child for the couple, and Aldean is also dad to two daughters, Keeley and Kendyl, from a previous marriage.
Brittany and Jason welcomed baby Memphis in December 2017, and Jason celebrated his first Father's Day as a dad of three in June.
He marked the occasion with a photo of himself and his children, writing, "One of the greatest gifts in life has been getting to be the dad to these 3 kiddos. I also had a pretty good teacher to learn from!! Happy Fathers Day to my dad @bigdawg1953 and all the other dads out there."
Brittny and Jason married in March 2015 in Mexico.
"I hate to sound like a Hallmark card, but we truly are best friends," Jason said at a press conference in March, via The Boot. "We do everything together. She's on the road with me; I don't like being out there unless she's out. We're kind of inseparable, and I like it that way. It's cool when you can be married to someone who you're truly best friends with. That makes it fun, and makes it the way it's supposed to be."
"She's a great mom," he said of his wife. "Her favorite thing is bath time. She would give him five baths a day just because she likes to see him in the tub. He likes getting baths, anyway. That's their little special part of the day."
The 41-year-old added that he thought he was finished having kids until Memphis arrived.
"I'm excited to have a boy," he shared. "After all this time, and after honestly thinking I was probably done having kids, for him to come along is pretty cool."
Since Memphis' birth, Brittany and Jason been gushing over their baby boy on social media, with the pair often sharing photos and videos of their son.
In June, Brittany shared a clip of Memphis working on his swimming lessons, with Jason holding his son in a pool as the infant splashed around, clearly loving every second.0comments
"Little water baby," she wrote.
Photo Credit: Getty / Jeff Kravitz