Kourtney Kardashian impressed fans with her post-baby body this week on Instagram.

The 36-year-old reality star showed off her trim figure in a tummy-baring selfie posted Thursday. The photo received over 718,000 likes, including from pop star Justin Bieber, who is friends with Kardashian's half-sister Kendall Jenner.

"Hopped out the bed, turn my swag on Took a look in the mirror said what's up?" she captioned the picture, quoting Soulja Boy song "Turn My Swag On."

Kardashian's selfie comes just six months after welcoming her third child, son Reign, with longtime partner Scott Disick. She and the 32-year-old club promoter have been together since 2006, and are also parents to five-year-old son Mason and two-year-old daughter Penelope.

Kardashian's personal trainer, Don Brooks, told E! News the reality star lost weight using his Don-A-Matrix Method, saying, "Like most women the clothing test tells all -- so I love when she says [she's] now able to fit into her favorite jeans or shorts again!"

Kardashian and her family currently star on E! series Keeping Up with the Kardashians, which will return from midseason break in the fall. The reality star's younger sister, Kim Kardashian, is currently expecting her second child with husband Kanye West.