Kardashian West confirmed via Twitter that the baby boy had arrived Friday.
"He's here and he's perfect!" she tweeted.
The reality television star said the baby resembled his sister Chicago, and called him her "twin."
"I’m sure he will change a lot but now he looks just like her," she said.
The boy, whose name was not immediately disclosed, was delivered via surrogate.
On Thursday, Kourtney Kardashian had announced during an appearance on The Ellen DeGeneres Show that her sister's surrogate was in labor.
Kardashian West, 38, and West, 41, have been married since 2014 and have three other children: North, 5; Saint, 3; and Chicago, 1.
Kardashian West began going the surrogate route after dealing with a host of issues during her first two pregnancies.
With the couple's eldest child, North, Kardashian experienced appendicitis and preeclampsia, a serious condition that causes high blood pressure and sometimes other symptoms like water retention. The condition can make the birthing process difficult for the mother and baby.
A couple of years later, Kardashian West experienced another trying pregnancy with Saint. The reality star developed placenta accreta, another serious ailment that causes the placenta to grow too close to the uterine wall.
On Kardashian West's television show, Keeping Up With the Kardashians, she went into detail about how her doctor removed the rest of her placenta using his hands.
In a recent interview with Vogue magazine, she answered a few questions about being a parent.
"You just have to wing it," she said after being asked what her secret to being a great parent was.
She also revealed that, as of April, the room for baby number four wasn't complete, so she was "kinda stressing."
And more children could be coming in the future. People magazine reported in 2017 that she and her husband were open to having "five or six" children in total.