The announcement was made by a spokesperson for Meghan and Prince Harry. They 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.”
The announcement was accompanied by a photograph of the couple posted on Twitter by Misan Harriman, a long-time friend.
The black and white image shows Meghan resting her head in Harry’s lap with one arm over her clearly pregnant belly.
Accompanying the photo Mr Harriman said: “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!”
Fans of the couple took to Twitter to celebrate the surprise announcement.
Mandy Thrall, from south-west England, posted: “So beautiful and so very much in love.
“They must be over the moon for the new baby.”
READ MORE: Meghan Markle pregnant – What will the royal baby be named?
When born the new royal will be eighth in line to the British throne.
Meghan gave birth to the couple’s first child, Archie Mountbatten-Harrison, in May 2019.
In July last year Meghan tragically had a miscarriage which she wrote about movingly for the New York Times.
The Duchess explained she was changing Archie’s nappy when she “dropped to the floor” in pain.
She wrote: “I knew, as I clutched my firstborn child that I was losing my second.
“Hours later, I lay in a hospital bed, holding my husband’s hand.
“I felt the clamminess of his palm and kissed his knuckles, wet from both our tears.
“Staring at the cold white walls, my eyes glazed over. I tried to imagine how we’d heal.”
Meghan and Harry stood down as senior members of the Royal Family in March last year.
They moved to California, Meghan’s home state, shortly before coronavirus lockdown was imposed.
During the shutdown both royals were spotted delivering supplies to vulnerable residents in Los Angeles.