The announcement was made ahead of the royal baby's christening in Norfolk.
Other godparents include Catherine's cousin Adam Middleton and Thomas van Straubenzee, a friend of the duke's.
The christening will be held on the Sandringham estate and attended by the Queen, other senior members of the Royal Family and Catherine's parents.
The duke and duchess, Prince George and the princess will appear as a family of four in public for the first time at the baptism at the Church of St Mary Magdalene in Sandringham.
The baptism, conducted by Archbishop of Canterbury the Most Reverend Justin Welby, will be held in private.
But members of the public will be allowed into the area outside the church - known as the paddock - to see Charlotte and other royals arrive and depart.
Prime Minister David Cameron tweeted: "My best wishes to the Duke and Duchess of Cambridge on Princess Charlotte's Christening today."