"The Duke and Duchess of Cambridge are very pleased to share a photograph of Prince George and Princess Charlotte at Kensington Palace this morning," the caption read.
"The photo was taken shortly before Their Royal Highnesses left for Thomas's Battersea."
Charlotte joined her big brother Prince George (we still remember his first day!), who was more than happy to show her the ropes at the elite British institution.
British reporter Rebecca English shared, "She’s very excited, I’m told - as is George at the chance to show her around the school where he has been a pupil for the last two years."
Just like with Prince George's first day, Kensington Palace shared official portraits from the big day.
Kate asked teacher Ms Haslem, (who was there for George on his first day), ''How are you? Did you have a nice summer?''
Earlier this year, the Palace announced that Princess Charlotte, who was born on May 2 2015, would be embarking on her big new milestone with the following statement:
"Princess Charlotte will join Prince George at Thomas's Battersea from September 2019 this year."
Simon O'Malley, Headmaster at Thomas's Battersea echoed the family' excitement.
He said: "We are delighted that The Duke and Duchess of Cambridge have decided that Princess Charlotte will join her elder brother, Prince George, at Thomas's Battersea."
"We greatly look forward to welcoming her and all of our new pupils to the school in September."
WATCH: Princess Charlotte throws tantrum at end of royal visit