The couple welcomed a baby girl named Eva James Jenner last Wednesday (22 July), but only announced the news via Instagram on Sunday.
Leah said: "On Wednesday, July 22nd I became a Mum. It has without a doubt been the most profoundly beautiful time in my life but also emotionally, spiritually and physically challenging too....and I am so grateful for every second of it.
"Brandon and I are taking care of our heathy, little baby girl, Eva James Jenner."
Brandon, who is Kim Kardashian's stepbrother, announced in March that he and Leah were expecting their first child together.
The couple, who perform as indie pop duo Brandon & Leah and have appeared on Keeping Up With The Kardashians, married in Hawaii in 2012.
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