The couple shared their happy news in a statement issued on the reality tv star's website.
"Kanye and I are happy to announce the arrival of our healthy, beautiful baby girl," Kim wrote in a post titled 'She's Here!'
"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."
The new arrival will join big sister North, four-and-a-half, and big brother Saint, two.
"North and Saint are especially thrilled to welcome their baby sister," the couple added.
Kim, 37, and Kanye, 40, haven't yet revealed the new arrival's all-important moniker -- although, rumour has it the eldest of the bunch, North, will pick out the little's name.
A source recently told Hollywood Life, "Kim and Kanye are very concerned about how North is going to react to the new baby. It's especially challenging because they're using a surrogate so they can't even really prepare her."
The insider added: "Kim has called on a parenting expert to get advice on what to do to avoid sibling rivalry this time around and one of the suggestions was to let North help name the baby so she feels more invested."
WATCH: Kim reveals she and husband Kanye will expand their brood. Post continues…
A source had previously confirmed to People that the mega-famous family had hired a surrogate to help expand their family.
"The entire family is over the moon. Kim had been looking for a surrogate for months until recently when she found the perfect candidate," a source told the publication in September.
"Given her health scares in the past, Kim felt the need to hire a surrogacy agency that helped serve as the liaison in finding a healthy woman who would be a great surrogate option for her and Kanye," the source added. "Both of them have been super involved in the process."
The beauty mogul suffered from preeclampsia during her first pregnancy as well as a life-threatening condition called placenta accreta, in which the placenta remains attached to the uterine wall instead of coming out after the birth of the baby. The same complication presented itself during her second pregnancy.
Explaining the experience on her website after the birth of her second-child, Kim wrote that her doctor "had to stick his entire arm in me and detach the placenta with his hand, scraping it away from my uterus with his fingernails."
She added: "My mom was crying; she had never seen anything like this before. My delivery was fairly easy, but then going through that—it was the most painful experience of my life! They gave me a second epidural but we were racing against time, so I just had to deal."
Congratulations to the happy family-of-five!