There had been little indication that the 35-year-old Kate, the Duchess of Cambridge, was pregnant.
The royal couple already have two children - Prince George and Princess Charlotte - aged four and two.
George is set to start school in London this week and Charlotte celebrated her second birthday on May 2 this year.
"The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child," Kensington Palace said in a statement.
More From This Section
Kate is suffering from severe morning sickness or hyperemesis gravidarum as in the early stages of her previous two pregnancies. Her condition has forced her to cancel a planned engagement at a children's centre in London today.
"The Duchess is being cared for at Kensington Palace," the statement noted.
George will be attending the Thomas' Battersea school near the royal couple's Kensington Palace home in the city.
Prince William had resigned from his job as an air ambulance rescuepilot in July to take on royal duties full time and spend more time with his family.
The second in line to Britain's throne is also set to take on additional royal responsibilities as his grandfather and the Queen's husband, PrincePhilip, commenced his retirement from royal duties last month.
Queen Elizabeth II andPrincePhiliphad already cut down on long-haul travel for some years now, with younger royals taking on those duties.