The Keeping Up With The Kardashians stars announced their happy news via Instagram on Sunday evening (15 March), alongside a photograph of Leah's blossoming baby bump.
Leah's caption read: "Well, it's time we share the good news. We're having a baby."
Brandon is the son of Bruce Jenner, who was married to Kris Jenner, the mum of Kim, Khloe and Kourtney Kardashian.
His brother is former The Hills star Brody Jenner and his half sisters are Bruce and Kris' kids Kendall, 19, and Kylie, 17, Jenner.
Brandon and Leah have been featured on the Kardashian family reality shows in the past.
They are singers who perform as a duo, Brandon & Leah.
Brandon and Leah married in Hawaii in 2012, with Kendall and Kylie serving as bridesmaids. Brody was best man. The Kardashian sisters did not attend but sent their well wishes via Twitter.
Leah is the daughter of former Eagles guitarist Don Felder.
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