“This little baby girl will make us a family of five,” the former Newlyweds star, 38, said in a post on Instagram.
“We couldn’t be happier to announce this precious blessing of life.”
The singer-turned-designer posted two photos on Instagram of daughter Maxwell, 6, and son Ace, 5, with pink gender-reveal balloons, Page Six reported.
The news likely comes as a surprise for fans of Simpson and her husband, former NFL tight end Eric Johnson, 39.
Last May, the former Fashion Star mentor made a now-infamous appearance on The Ellen DeGeneres Show where she swore she wasn’t having any more kids.
“I’m not pregnant … We had an IUD,” Simpson insisted, referring to a birth control implant.
“Nothing’s gonna get in that uterus.
“I have two beautiful children. I’m not having a third.”
In 2015, Simpson told Entertainment Tonight she was in no rush to expand her family with Johnson
“No more kids on the horizon,” she said at that time, according to Fox News.
“That would freak me out a little bit.”
View this post on Instagram
A post shared by Jessica Simpson (@jessicasimpson) on
The former reality star also recently opened up about her marriage to Johnson, who she said is the secret to her youthful look.
“He keeps me hot, I have to say,” Simpson told ET in July.
“He’s always throwing around the kids, and he’s just an amazing father, it makes me want to stay young.
“Like how can you be so active? I mean watching the kids running makes me exhausted.”
Simpson’s main career now is running her brand, Jessica Simpson Collection — a line of clothes, shoes, handbags and beauty products.
The hugely successful business, which also includes perfume, rakes in $US1 billion ($A1.38 billion) annually.