“We can confirm that Archie is going to be a big brother,” a spokesperson for the couple said.
“The Duke and Duchess of Sussex are overjoyed to be expecting their second child.”
The couple shared a black-and-white picture of themselves, sat under a tree, with Harry resting his hand on Meghan’s head as she lies in his lap cradling her bump.
Their new baby will be eighth in line to the British throne, one step down from their first child, Archie Harrison Mountbatten-Windsor, who was born in May 2019.
A Buckingham Palace spokesman said: “Her Majesty, Duke of Edinburgh, Prince of Wales and the entire family are delighted and wish them well.’’
“I knew, as I clutched my firstborn child, that I was losing my second,” she wrote.
The couple quit their roles as senior working royals last year and now live in an £11m house in Montecito in California.
Harry retained his place in the line of succession despite quitting royal life, and dropping his HRH style.
When Harry and Meghan’s new child arrives, the Duke of York, who was born second in line in 1960, will move down to ninth place.
The first seven places will remain unchanged – the Prince of Wales, the Duke of Cambridge, Prince George, Princess Charlotte, Prince Louis, Harry, and Archie.
The Duke and Duchess of Sussex announced they were expecting their second child days after Meghan won a privacy case against the Mail on Sunday over the publication of a “personal and private” handwritten letter to her estranged father.
Additional reporting by Press Association