"Our baby boy, Crew Gaines, is here & we couldn't be more in love," Gaines wrote on Twitter, alongside four photos of the family in the hospital.
"He made an unexpected (and speedy) entrance into the world two and a half weeks early - which is fitting given he was a sweet surprise from day one. Thank you for all your thoughts and prayers. We are so grateful."
Gaines also shared a photo of their four other children waiting outside her hospital room to hear Crew's birth.
"One of my favorite pics a friend showed me after Crew was born. They couldn't wait to meet their new baby brother," she wrote.
One of my favorite pics a friend showed me after Crew was born. They couldn't wait to meet their new baby brother ❤️ pic.twitter.com/NvFvOFPK0p— Joanna Gaines (@joannagaines) June 24, 2018
"And then there were 5," Chip tweeted. "The Gaines crew is now 1 stronger! 10 beautiful toes and 10 beautiful fingers all accounted for, and big momma is doing great!"
In a May interview, the 40-year-old Gaines said she was excited about having another baby, eight years after her previous pregnancy.
"I had been feeling a little emotional and crazy and thought, 'Wow, this isn't me,'" she told PEOPLE. "Then, all of a sudden I realized, 'Hey, I remember this.' When I saw the results of the test, my jaw dropped. Chip was in the driveway about to leave when I called him back and said, 'We're pregnant!' He was so excited. We're both just so excited."
Gaines also said her older children were excited, too. They wanted to have a part in welcoming their new sibling into the world.
"Every single time they see my belly they have to rub it. I think it's sweet because they're older now, and I can see them wanting to be a part of this," Joanna said. "We're all rallying around this baby, which I feel is a sweet gift to our family."
The Gaines are already parents to Emmie Kay, 8; Duke, 9; Ella, 11; and Drake, 13. The couple starred in HGTV's Fixer Upper, which ended in April so they could focus on their restaurants and other business interests.
Photo credit: Twitter/Joanna Gaines