The reality TV star confirmed her second son's birth via Twitter on Friday, May 10, saying: "He’s here and he’s perfect!"
"He’s also Chicago’s twin lol I’m sure he will change a lot but now he looks just like her," she added.
There's no news yet on the baby's name.
The rapper and the makeup mogul are already parents to North, 5, Saint, 3 and Chicago, 18 months. The surrogate reportedly went in labor on Thursday, according to Kourtney Kardashian, who spilled the beans on "The Ellen DeGeneres Show."
Their mother, Kris Jenner, was a guest on the talk show when the Poosh founder revealed, "My mom doesn't even know this, but Kim's surrogate went into labor. So, she's at the hospital."
Jenner was shocked and said, "No! What are we doing sitting here?! Come on, Ellen!"
Kardashian announced she was expecting baby number four in January and has been open about her decision to use a surrogate because of health complications.
She wrote on her website, "Last pregnancy, I had a condition called preeclampsia, which is a serious condition you can get during pregnancy; often, the only way to get rid of it is to deliver early to protect yourself and the baby. Only about 5 percent of women get this. Lucky me! It causes your body and face to swell, and that was very uncomfortable for me. I had early-onset preeclampsia and I had to deliver at 34 weeks, almost six weeks early."
"We induced my labor and I had North soon after. Right after delivery, the placenta usually then comes out. Mine did not. My placenta stayed attached inside my uterus, which is a condition called placenta accreta. This is a high-risk condition that happens when the placenta grows too deeply into the uterine wall."
Due to the serious risk, Kardashian decided not to carry her third and fourth children.