Viewers of last night's Keeping Up With The Kardashians in the US saw her make the big announcement in a teaser at the end of the episode.
Kourtney, 35, sprang the news on long-time boyfriend Scott Disick, 31, while she was tidying up her wardrobe.
"I'm pregnant," she said, adding: "This was not a plan."
Scott's response? "Holy s**t."
Kourtney's baby news came following intense rumours that she was expecting a third child.
She and Scott are already parents to son Mason, four, and daughter Penelope, one, whose births have both been documented on the show.
Kourtney's pregnancy announcement was made during a collage of teaser clips promoting the rest of the new series of Keeping Up With The Kardashians, which will include - of course! - sister Kim's wedding to Kanye West.
So far, there are no other details about Kourtney's pregnancy, such as a due date or whether she's expecting a boy or girl, but we're assuming we'll learn all of that in upcoming episodes of the show.
Keeping Up With The Kardashians returned for its brand new series in the US last night. We'll get the premiere episode over here in the UK on Sunday night on E!
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Celebrities who announced pregnancies in 2014 in pictures:
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