And no, it didn't involve party poppers and a Colin the Caterpillar cake.
The reality star and model marked achieving 40 million followers on the photo-sharing app by flashing her boob for the world to see.
Pulling down her top in a racy black and white snap, the picture would have been considered totes inappropes by the Insta police if it weren't for the red X hovering over her nipple.
The 20-year-old told fans: "My favourite photo for all 40 mil of you. genuine happiness...taken by lil Ky."
She then tagged friend Moises Arias in the message, confusing us as to whether Moises or her little sister Kylie took the topless photo.
Miss Jenner is now just trailing behind sisters Kylie, who has 41.4m followers, and Kim, who rules the roost – and indeed all of Instagram – with a whopping 51m followers.
While Kendall is usually the least shocking of the Kardashian family, she has amped up the sex appeal in her latest pics.