Kardashian, 30, broke the news on the NBC "Today" show while she and other family members were promoting their latest reality series, "Kourtney and Khloe Take Miami," Todayshow.com reported.
Kardashian named Scott Disick as the father of the baby due in December. Viewers of "Keeping Up with the Kardashians" followed Kardashian's troubled relation with the New York playboy during the show's two seasons.
"It's Scott. My ex-boyfriend. Who is now my boyfriend," she said.
The couple split in January but have seen each other on occasion, Todayshow.com said.