The 36-year-old made the revelation in a trailer for the new series of her family reality show, Keeping Up With The Kardashians.
A short clip shows Kim having a Facetime conversation with her sister, Khloe, whom she asks: "What happens every time I say, 'Guess what?"'
Khloe replies: "You're pregnant or the person's pregnant?"
To which Kim excitedly confirms: "We're having a baby!"
Kylie Jenner owns a lucrative cosmetics line
Kim and Kanye West, 40, are expected to use a surrogate for the birth.
Health complications during the pregnancies of their first two children - North, four and Saint, 21 months - have previously led Kim to say: "I feel like surrogacy really is the only other option for me".
This latest news means that there may now be three Kardashian siblings with babies on the way.
Unconfirmed reports over the last week suggest that the youngest of the Kardashian-Jenner siblings - Kylie Jenner, 20 - is pregnant for the first time with rapper boyfriend Travis Scott.
And Kim's sister, Khloe, 33, is also understood to be expecting her first child with Canadian basketball player Tristan Thompson.
Kylie and Khloe have neither confirmed nor denied reports.
The 14th season of KUWTK starts on Sunday on E!