Prince William and his wife Catherine say they are "delighted to confirm" their third child will be due in April next year.
The Duke and Duchess of Cambridge had already revealed they were having a child, but did not previously say which month the child is due.
The brief Kensington Palace announcement came after the Royal couple attended a charity event at Paddington Station in London earlier this week.
The couple were forced to go public with the pregnancy when Catherine again suffered from hyperemesis gravidarum — acute morning sickness — and had to cancel a planned engagement.
The Duchess also suffered from the condition with her first two pregnancies.
William and Catherine, both 35, already have two children: Prince George and Princess Charlotte, who are aged four and two respectively.
Their third child would be fifth in line to the British throne and if the child is a boy, his arrival will be historic because he would not displace his big sister in the line of succession.
William is a grandson of the Queen and the eldest son of Prince Charles, who is the first in line to the throne.