The 35-year-old was seen breaking the news to her long-term partner Scott Disick at the end of the episode, in a montage of clips showing viewers what to expect during the forcoming new season. "I'm pregnant," she says. "It was not a plan."
Kourtney Kardashian and Scott Disick are expecting their third baby together
Kourtney and Scott have been together for seven years and are already parents to Mason, four, and Penelope, 23 months. They will welcome their third baby together in December.
It was first reported last week that the TV star is pregnant, with a source telling US Weekly: "Kourtney wants to have a handful of kids."
A friend confirmed that the couple don't know the gender yet, adding: "They just want the baby to be healthy."
Kourtney is currently filming spin-off show Kourtney & Khloe Take The Hamptons.
Kourtney Kardashian with son Mason, daughter Penelope, and partner Scott Disick
Her mum Kris Jenner and sister Kim Kardashian had both confirmed to fans that the pregnancy would be addressed on Sunday's show.
"Kourtney will also be addressing pregnancy rumours this Sunday on our premiere!" Kim wrote on Twitter. "So be sure to tune in! #keepingupwiththeKardashians."