Princess Sofia is understood to be in good health and is expected to give birth in September 2017.
"We are pleased to announce that we are expecting a child, Prince Alexander's sibling. We look forward to welcoming a new little member of our family," she and her husband said in a statement published by the Royal Court on Thursday.
Alexander, their first child, is the youngest member of Sweden's growing royal family. He was born in April last year.
The new baby will also have plenty of royal cousins to play with. Prince Oscar, the second child of Princess Victoria, was born just seven weeks before Alexander. He is the younger brother of five-year-old Princess Estelle. London-based Princess Madeleine also has two children, Princess Leonore and Prince Nicolas.
Carl Philip, 37, is the second child and only son of King Carl XVI Gustaf and Queen Silvia of Sweden. His elder sister Princess Victoria is first in line to the throne.