At Prince Louis' christening this afternoon at St James's Palace, the three-year-old was spotted hand-in-hand with her parents as she made her way into the chapel for the service.
Newlyweds Prince Harry and Meghan, who looked stylish in an olive-green Ralph Lauren outfit and Stephen Jones hat, were also in attendance, along with Louis' siblings Prince George, 4, and Princess Charlotte, 3.
While many famous royal faces will gather for Prince Louis' christening today, Queen Elizabeth II won't be one of them.
She will attend high profile celebrations in central London marking the centenary of the RAF on Tuesday, and a visit by U.S. president Donald Trump in Windsor on Friday. Prior to the christening William and Kate announced six friends and family members to be Louis' godparents.
For the special day, the Archbishop of Canterbury was on hand to baptise 11-week-old Prince Louis.
The baby wore a handmade replica of the Royal Christening Robe, which was made in 1841 for the christening of Queen Victoria's eldest daughter, according to Kensington Palace. He was born April 23, 2018.
Who are Prince Louis' godparents?
William and Harry wore matching blue suits.
Middleton, who is now five-months pregnant with her first child with husband James Matthews, hid her baby bump under the long-sleeved pleated gown.
Louis' three godfathers will be family friend Nicholas Van Cutsem, William's school friend Harry Aubrey-Fletcher and Guy Pelly, a well-known high society nightclub founder who has been close to William and brother Harry for many years.
Lucy Middleton: She is the Duchess of Cambridge's cousin and a solicitor.
Lady Laura Meade: Also one of Princess Charlotte's godparents, Lady Meade is the wife of Prince William's close friend and fellow Etonian James Meade, known as "Meadey".