Princess Anne's Canadian daughter-in-law will welcome the new arrival in March.
"The Queen and The Duke of Edinburgh, as well as Autumn's family, have been informed and are delighted with the news," says a representative.
The couple's first child, a little girl named Savanah, was born on December 29, 2010 at Gloucestershire Royal Hospital.
There will be just 15 months between the new baby and his or her big sister.
Peter, 33, and Autumn – who is the same age – wed in May 2008. The happy occasion was covered exclusively by HELLO! magazine.
At the time, Autumn spoke to us about her and Peter's desire to start a family.
"Hopefully we'll be blessed with at least one child," she said. "I don't want to hand it over to somebody else. I'd like to raise my own children."
After the couple's wedding, they moved to Hong Kong, but returned to the UK before Savannah was born.
They now live in London, where Peter – who is 11th-in-line to the throne – works for the Royal Bank of Scotland.
For the full story, see this week's HELLO! magazine – isssue 1197 – on sale now.
It's a little princess!