The American actress and singer, 38, revealed on Instagram she and husband of four years Eric Johnson will welcome a daughter.
Simpson announced the news by sharing a picture of her two children, six-year-old daughter Drew and five-year-old son Ace, holding large balloons containing smaller pink ones.
She later posted an image of her cradling her baby bump.
Simpson said: “SURPRISE… This little baby girl will make us a family of five. We couldn’t be happier to announce this precious blessing of life.”
Simpson found fame as a teen pop star in the late 1990s before starring in MTV reality series Newlyweds: Nick And Jessica with her first husband Nick Lachey.
The show ended in 2005, with their marriage following shortly afterwards.
Simpson has since starred in films including The Dukes Of Hazzard and Employee Of The Month.