Princess Charlotte with Prince George have been pictured together outside school in an adorable snap released by Kensington Palace.
Charlotte, four, started school at Thomas Battersea's in south-west London.
The Cambridge family were earlier greeted by Helen Haslem, head of lower school, as William said his daughter was "very excited".
The Duchess had some encouraging words for Charlotte as they walked through the school doors after being greeted by their teacher, and they were about Prince George.
In a video showing the family arriving for the special day, Kate can be heard saying: "George will have to show you the way."
Charlotte played with her ponytail as her parents shook hands with Ms Haslem.
As they walked William told Ms Haslem: "First day - very excited."
Kate said: "How are you? Did you have a nice summer?"
The four-year-old has been attending Willcocks Nursery in Kensington since last January but she's now making the exciting jump to big school with her brother.
Sources told the Daily Mail that George is "excited and proud" to be showing her around.
His sister will be known to staff and schoolmates as Charlotte Cambridge.
The Cambridges will be entitled to a discount for having a second sibling at the school, with annual fees reduced from £19,287 to £18,915.
