Princess Charlotte is "very excited" about starting school, the Duke of Cambridge said as he dropped her off for her first day.
Walking across the playground with both parents and her brother, Princess Charlotte smiled as she met the head of the lower school at Thomas's Battersea.
Prince George has attended the private school in south west London since 2017.
He began his first day of year two - his final year in the lower school before he moves to the middle school.
The Duke and Duchess of Cambridge released a photograph of Princess Charlotte and Prince George taken outside Kensington Palace before they left for St Thomas's.
And as she arrived at the school, Princess Charlotte, four, was photographed by the press meeting head teacher Helen Haslem.
Her uniform includes a navy pleated skirt and cardigan, white socks and black shoes.
The duchess carried her daughter's backpack, which was decorated with a pink key-ring in the shape of a pony's head. She smiled as she greeted Ms Haslem and asked about her summer holidays.
Ms Haslem bent down to shake hands with both George and Charlotte, who is fourth in line to the throne.
The duke and duchess accompanied Charlotte to her classroom before saying their goodbyes.
Prince William, who drove the family to the school, said: "First day - she's very excited."
Prince George was also photographed on his first day at Thomas's Battersea, a preparatory school located a few miles from the family residence in Kensington Palace, in September 2017.
He, too, was greeted by Ms Haslem, after Prince William drove him through the school gates. The Duchess of Cambridge missed the occasion as she was not well enough to take him.
The school has around 560 pupils between the ages of four and 13.
It charges £6,429 per term for a family's eldest child and £6,305 for their second eldest child throughout reception, year one and year two, according to its website.