The actress and her husband Ben Affleck - who already have two daughters, Violet, five, and two-year-old Seraphina, together - are "thrilled" about their impending arrival, according to statement released to the Associated Press.
Although no other details about the pregnancy have been released, it is believed Jennifer is in her second trimester.
Sources claim the couple have been trying for a baby for a while as they have been keen to extend their family.
One said: "She`s always wanted three kids. This was her plan all along."
Jennifer, 39, has previously spoken out about motherhood, saying there is no right way to raise children and does things to suit her family.
She said: "It`s tricky, I don`t think there`s any right way to be a mom. Whatever works for you, don`t judge yourself for it. And don`t let others judge you. Just do what feels best for your family, and if that`s going to work, then go to work with your head held high, and if that`s staying at home, then don`t let yourself feel any less for being at home with your kids."