The brunette beauty, who has rocked long locks down to the bottom of her waist for what feels like forever, chopped off about a foot of her hair into a shoulder-length bob.
In the Instagram photo she posted, Kourtney was wearing her hair down with a side part as well as a cropped white T-shirt -- with what appears to be no bra -- and mid-rise light blue jeans. She posed for a mirror selfie in a bathroom, with the toilet paper dispenser visible in the background.
Kourtney was definitely going for a sexy look, as her nude-colored thong was poking out of one side of her jeans.
"Don't worry, I ordered a new phone case last night," Kourtney captioned the photo, referencing her case that features a swirly pattern of Kimoji crying faces on top of turquoise, pink and yellow tie dye.
Kourtney's makeover is a huge change for the reality TV star and mother of three. She has always been the least experimental of her sisters when it comes to her look, other than model Kendall Jenner.
Kim Kardashian, for example, has sported white platinum hair recently and all different hair lengths, while Kylie Jenner has pretty much tried every color of the rainbow, from blue to candy-apple red.
Kourtney may have been inspired to change her appearance by boyfriend Younes Bendjima, a 24-year-old model and former boxer whom she's been dating for months now.
Kourtney and her ex-boyfriend Scott Disick are parents of three children: Mason, 7, Penelope, 4, and Reign, 2.
Kourtney and Scott's romance was on and off for almost a decade due to Scott's partying habits, and in Summer 2015, things ended for good when he was spotted getting touchy-feely with an ex, Chloe Bartoli, in Monte Carlo.