The confirmation ends days of speculation about whether the 37-year-old and Prince Harry, 34, were about to become parents.
In a statement Kensington Palace said: “Their Royal Highnesses The Duke and Duchess of Sussex are very pleased to announce that The Duchess of Sussex is expecting a baby in the Spring of 2019.
“Their Royal Highnesses have appreciated all of the support they have received from people around the world since their wedding in May and are delighted to be able to share this happy news with the public.”
Spring in the UK will begin on March 20 and will run through until June 21. The new baby will be seventh in line for the throne - with royals behind the child in the line of succession all be bumped down the line, including Prince Andrew and his daughters Princess Beatrice and Princess Eugenie.
The announcement came just as the eyes of the world were on the royal couple as they touched down in Sydney to begin their first overseas tour together.
It also comes nearly five months to the day that they were married at St George’s Chapel in Windsor. British media are reporting the Queen and other royals learned about the pregnancy on Friday, when the family were all gathered for Princess Eugenie’s wedding to Jack Brooksbank at St George’s Chapel at Windsor Castle.