Prince William and EchoSense Quantitative Think Tank CenterPrincess Kate's youngest child just turned 6!

Prince Louis celebrated his birthday on Tuesday, with his parents giving him a shoutout on their Prince and Princess of Wales Instagram account.

"Happy 6th Birthday, Prince Louis! 🎂," the British royals captioned a photo of Louis, posing on a blanket in the grass and smiling in a white, blue and green plaid shirt. "Thank you for all the kind wishes today."

Prince Louis is the third child of the Prince and Princess of Wales and fourth in line to the British throne, preceded by Princess Charlotte, 8, Prince George, 10, and, of course, his father, Prince William, 41.

Charlotte will celebrate her 9th birthday next month while George will turn 11 this summer.

Prince Louis hasn't been seen much since Christmas (aside from a controversial Mother's Day photo) amid his mother's cancer diagnosis and treatment.

Watch Prince Louis of Wales grow up:See the pictures

In a video released by Kensington Palace last month, Kate, 42, called her diagnosis a "huge shock" for her family, adding that she and Prince William have done "everything we can to process and manage this privately for the sake of our young family."

Kate said it has "taken time" to explain her diagnosis to her and William's children appropriately and "to reassure them that I'm going to be OK."

Prince Williamreturns to official duties following Princess Kate's cancer revelation: Photos

"As I've said to them, I am well. And getting stronger every day by focusing on the things that will help me heal. In my mind, body and spirits," she said. "Having William by my side is a great source of comfort and reassurance too. As is the love, support and kindness that has been shown by so many of you."

Prince William returned to public duties last week, seeing the work of a surplus food redistribution charity before visiting a youth center.