Simpson and Johnson had a little help announcing their news from their two kids, 6-year-old daughter Maxwell and 5-year-old son Ace.
In a second Instagram post, pink balloons were revealed with her kids celebrating in their backyard.
Before adding: 'This little baby girl will make us a family of five.
Jessica and former National Football League player Eric got engaged in 2010, before getting married in front of their children and family in July 2014.
Many of her 4.3 million Instagram followers took to the comments section on both posts, letting loose their excitement for baby no. 3.
"Aww wow, congratulations to you and your family xx", someone else said.
The singer and fashion designer announced on social media Tuesday that she's pregnant with her third child. It looks like you have two very excited older siblings!' "Get ready for some serious heart melting!" "We always practice", Simpson said coyly. Just past year, she told Ellen DeGeneres that she was taking precautions to avoid another pregnancy. "He's got such good tone", she said.
In April, Simpson did hint to ET that she wanted another baby. "But it would definitely have to be a miracle".
But a year ago, on "Ellen", Simpson said, "We got an IUD, nothing's gonna get in that uterus".
She said: 'Being a mother is the best opportunity and challenge I've ever been blessed with in life.