Sweden's Prince Carl Philip, who is fourth in line to the throne, and his wife Princess Sofia are expecting their second child, the palace announced on Thursday.
"The birth is expected to take place in September 2017," it said in a statement.
Sofia, 32, and Carl Philip, 37, married in June 2015 and have a son, Alexander, who was born in April 2016.
"We are looking forward to welcoming a new little member to our family," the couple said in the statement.
Carl Philip is the second child of King Carl XVI Gustaf and Queen Silvia. His elder sister Victoria, who has two young children, is heiress to the throne.
Princess Sofia is hugely popular among Swedes for her down-to-earth manner and the relatively normal lifestyle she and Carl Philip live.
A former contestant on TV reality show "Paradise Hotel", Sofia famously posed topless with a boa constrictor in a men's magazine when she was 20.
She later studied accountancy in New York and became a yoga teacher, while also working as a waitress before returning to Sweden.
She then headed a humanitarian organisation for disadvantaged children in South Africa that she founded in 2010 after earlier African charity work, but gave up the role after her marriage to focus on royal duties.