A spokesperson for the royal couple said: "We can confirm that Archie is going to be a big brother. The Duke and Duchess of Sussex are overjoyed to be expecting their second child."
Sharing a black and white picture of themselves under a tree with Harry resting his hand on Meghan's head as she lies cradling her bump, the couple sent social media into a tizzy with the good news.
Misan Harriman, a friend of the couple and the photographer who took the photo that accompanied their announcement, tweeted: "Meg, I was there at your wedding to witness this love story begin, and my friend, I am honoured to capture it grow. Congratulations to the Duke and Duchess of Sussex on this joyous news!"
The Buckingham Palace, responding to the news, said: "Her Majesty, Duke of Edinburgh, Prince of Wales and the entire family are delighted and wish them well", The Guardian reported.
The Sussexes' baby will be the Prince of Wales' fifth grandchild and the Queen and the Duke of Edinburgh's 10th or 11th great-grandchild, depending on whether he or she arrives before or after Zara Tindall's baby, which is also due in 2021. He or she will be eighth in line to the throne and entitled to be a Lord or Lady, but the duke and duchess are expected to again opt for Master, like Archie, or Miss, with the surname Mountbatten-Windsor, the Guardian report explained.
Harry retained his place in the line of succession despite him and Meghan stepping back as senior royals in January last year, and dropping their HRH style.
The Duke of York, Prince Andrew, who was second in line when he was born in 1960, will move down to ninth place.
In November, Meghan disclosed the devastation that the couple experienced after she had a miscarriage last summer.
Speculation that the duchess might be expecting a second child started after she applied for and was granted a postponement of her privacy trial against the Mail on Sunday until the autumn for a "confidential" reason, the Guardian report said.
Any trial was averted when, on Thursday, the duchess won her high court privacy case against Associated Newspapers Ltd over the Mail on Sunday and Mail Online's publication of extracts from a private handwritten letter she wrote to her estranged father, Thomas Markle, 76, after the judge, Lord Justice Warby, granted summary judgment in Meghan's favour.
Harry and Meghan's announcement comes just five days after the royal family celebrated the arrival of Princess Eugenie's first child - a baby boy.
This new baby, like Archie, is set to grow up thousands of miles away from the Queen, Prince Charles, and Prince William, as the Sussexes now live in Montecito in California after leaving royal life in a quest for personal and financial freedom.
The couple have been busy securing funding for their new life, signing a lucrative deal with streaming giant Netflix rumoured to be worth more than 150 million pound, and another multi-million-pound podcast deal with Spotify.
Recently, Harry was seen shooting with TV presenter and fellow Brit, James Cordon, atop an open bus, maintaining Covid protocall. The photos went viral on social media.