"And then there were 5," Chip wrote with the hashtag #blessedBeyondBelief.
"10 beautiful toes and 10 beautiful fingers all accounted for, and big momma is doing great!"
The couple didn't share a picture of the newest little one or reveal his name, but Chip shared a video in March revealing the baby's gender.
They are parents to sons Drake and Duke and daughters Ella and Emmie.
In September, the couple announced that Season 5 of "Fixer Upper" is their last. The final episode aired in April.
The HGTV show, which premiered in 2013, featured the couple and their company, Magnolia Homes, as they remodeled more than 100 houses in the Waco, Texas, area.
Joanna designed the homes and Chip managed construction.