The image, taken by Catherine at Kensington Palace on Wednesday, shows a smiling Louis on a red bike, shortly before heading off for his first day at the Willcocks Nursery School.
The school, founded in 1964, is housed in a church hall just a short walk from Kensington Palace and was also attended by Louis' sister, Princess Charlotte.
It charges up to £10,200 (around $14,000) a year for morning attendance.
Louis was born on April 23, 2018, becoming fifth in line to the throne behind his older siblings, Prince George and Princess Charlotte.
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