The woman, who has not been named by the couple, welcomed their second daughter yesterday afternoon.
Kim, 37, announced the news in a sweet post on her website which simply read: "She's here! We're so in love."
She continued: “Kanye and I are happy to announce the arrival of our healthy, beautiful baby girl
“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.”
In her statement she revealed their baby girl weighed 7lbs 2oz and said she was doing well.
Kim confirmed she was pregnant on her show Keeping Up With The Kardashians.
Yesterday The Sun Online exclusively revealed that the baby was due "any day now".
The reality star has spoken publicly about her fears the couple would not be able to have another child after the first embryo implanted in the surrogate was lost.
Speaking to her pregnant sister Khloe during an episode of KUWTK, Kim explained: "It’s really nerve-wracking because there’s only a 60 percent chance it [the embryo] will take...
"I mean, I tried one and I lost it. So it’s just a chance."
She also explained that it took her and husband Kanye West a year to find the ideal surrogate to carry their third child.
Kim said it was a really “difficult situation” trying to find someone who would be the perfect fit for herself and Kanye .
She explained: “Getting a surrogate and finding someone that you really trust really is so much more for difficult of a situation than you could really imagine.
“You have to like make sure that their lifestyle is healthy and they have to go through psychological testing, and it was maybe a year before we finalised our current situation.
Last year Kim revealed she felt compelled to tell the surrogate that she was carrying her and Kanye's child - but although she claims she "loves her", she didn't invite her to her baby shower.
She said: "I just felt like I wanted, whoever's carrying my baby, like what if they weren't a fan of me or my husband? And what if they didn't want to be carrying our baby?
Kim and Kanye's first child was North, who turned four on June 15, 2017.
Their second child - son Saint - was born on December 5, 2015.
The revelation of Kimye's decision to use a surrogate came after Kim revealed she could "bleed to death" if she carried another child.