The reality star shared the sad news on her blog, saying she is heartbroken, reported Aceshowbiz.
"It is with deep sadness that I have to tell you all that my kitten, Mercy, has passed away. My heart is completely broken," she said.
Kardashian found out she's allergic to cat after she got Mercy. "I had never lived with a cat before and so I had no idea that I was allergic."
The kitten was then given to her sister Khloe's assistant who just lost her own cat.
A couple weeks later, Mercy got very sick due to "a stomach problem that we weren't aware of."
"Her stomach had filled with fluid and the doctor said she had a rare stomach cancer and that there was nothing we could do for her.
"Poor little Mercy died the very next day. It broke not only my heart that little Mercy died at only four months old, but Sydney's too," the 32-year-old TV star concluded.
"Mercy was a gentle and loving kitten and we are all going to miss her so much. Thanks to Sydney for making Mercy's short life one filled with love."