However, Middleton may have hinted at her pregnancy before the announcement during the couple's tour in Poland and Germany in July, PEOPLE reports.
While in Poland, Middleton was handed a newborn baby gift by a well-wisher, and she turned to her husband, saying, "We will just have to have more babies."
The royal couple, who is already parents to 4-year-old Prince George and 2-year-old Princess Charlotte, confirmed the exciting news in a statement from the Kensington Palace.
"Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child," the statement said.
"The Queen and members of both families are delighted with the news."
Middleton, 35, suffered acute morning sickness, also known as Hyperemesis Gravidarum, with her first two pregnancies. According to the statement, she is battling this condition during her third pregnancy as well.
Because of this, she canceled an appearance planned for Monday morning. "As with her previous two pregnancies, The Duchess is suffering from Hyperemesis Gravidarum. Her Royal Highness will no longer carry out her planned engagement at the Hornsey Road Children's Centre in London today," the statement said.
"The Duchess is being cared for at Kensington Palace."
Middleton is reportedly less than three months pregnant, but the palace made the announcement early due to her illness.
Prince Harry spoke out about the news, saying it was "fantastic" and "great." He is "very, very happy for them."
Photo Credit: Getty / Max Mumby