Buckingham Palace says Peter Phillips and his wife, Autumn, are expecting their second child, who will be Queen Elizabeth II's second great-grandchild.
The baby is due in March.
The palace said Friday that the Queen, her husband the Duke of Edinburgh and Autumn's family are delighted by the news.
The couple already have a daughter Savannah — born in December — who is 12th in line to the throne and was the queen's first great-grandchild.
Peter Phillips, the son of Princess Anne, is the queen's oldest grandchild. He is 11th in line to the throne, and married Montreal-born Autumn in May 2008 at Windsor Castle.