"She's here!" Kim titled her post. "We're so in love."
The couple's daughter was born via surrogate on January 15 at 12:47AM, weighing seven pounds and six ounces.
"Kanye and I are happy to announce the arrival of our healthy, beautiful baby girl," Kim revealed to the world.
"We are incredibly grateful to our surrogate who made our dreams come true with the greatest gift one could give and to our wonderful doctors and nurses for their special care. North and Saint are especially thrilled to welcome their baby sister. Love, Kim Kardashian West."
Kim and Kanye's new addition joins big brother Saint, 2, and big sister North, 4.
"North and Saint are especially thrilled to welcome their baby sister," Kim added in her website post.
Two days after news of the pregnancy broke, an insider told People that Kim and Kanye were expecting another baby girl and they were "thrilled" and "so excited."
Kardashian appeared to accidentally confirm her baby's sex on The Ellen DeGeneres Show in November when telling the talk show host that North was already calling her unborn sibling "Baby Sister."
Kim felt it was necessary to work with a surrogate because she had been told by doctors it would be too dangerous for her to conceive and attempt to carry another child.
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The reality TV star underwent two surgeries after delivering North in order to have her son Saint. She suffered from placenta accreta -- a condition in which the placenta stays attached to the inside of the uterus -- both times.
She had explained to her family that the operation was designed to repair her uterus and remove scar tissue, but there were complications during the procedure and it didn't work out as planned.
"[I feel] surrogacy is the only option for me," Kim said at the time. "I'm definitely leaning towards I want to try [surrogacy]. I always knew surrogacy was an option, now it's my reality. Whatever is meant to be will be."