Simpson and the former NFL player also share son Ace Knute, 5, and daughter Maxwell 'Maxi' Drew, 6.
The couple shared the sweet news to Instagram, writing: "We are so happy and proud to announce the birth of our perfect daughter, Birdie Mae Johnson. 3.19.19."
The couple first announced their pregnancy in August, with a sweet clip of their kids.
“SURPRISE … ” Simpson captioned an image of the couple’s children, as they prepared to pop two large balloons to find out the sex of the baby.
“This little baby girl will make us a family of five,” she added alongside a snap of her kids surrounded by smaller pink balloons. “We couldn’t be happier to announce this precious blessing of life.”
Fans were already privy to the name Simpson had chosen for her newborn daughter prior to her arrival, throwing a "Birdie's Nest" themed baby shower back in January.