"The Fam getting ready to surprise Kourtney for her birthday! Shhhh," wrote Kim next to the picture.
From left to right: Kris Jenner, Brandon Jenner, Leah Jenner, Kim Kardashian, Kendall Jenner, Bruce Jenner, Brady Jenner, Kylie Jenner, Lamar Odom and Khloe Kardashian
The whole family were involved in mum-of-two Kourtney's celebrations. Matriarch Kris Jenner joined her daughters Kim, Khloe, Kendall and Kylie as well as her step-sons Brody and Brandon Jenner. Brandon's wife Leah, Khloe's husband Lamar Odom and Bruce Jenner completed the party.
Kim also shared a snap of Kourtney's impressive birthday cake, a five-tiered replica of Hermes' famous gift boxes.
Earlier in the day Kourtney enjoyed more quality time with her family as the Kardashian clan went bowling.
The birthday girl was happy and relaxed at the party, dressed in a cute denim shirt and jeans, while her sister Khloe, 28, rocked a black skull-patterned bandana.
Reality star Kim, 32, swapped high fashion for a pair of ripped maternity jeans and a black blazer as she arrived at a bowling alley on Wednesday evening for the festivities.
Kourtney is the eldest of the Kardashian and Jenner siblings, and is happily settled with long-term partner Scott Disick, the father of her two children, Mason, three, and Penelope, nine months.