There had been plenty of interest about what the baby boy's name but the couple have a history of waiting a few days before revealing their children's names.
North, Saint, Chicago - and now Psalm.
"I'm sure he will change a lot but now he looks just like her", she tweeted. "So that's our one kind of name".
When capitalised, the biblical name refers to "the songs, hymns, or prayers contained in the Book of Psalms".
Back in January, Kim confirmed that they were expecting their fourth child during an appearance on Watch What Happens Live.
Speaking about her surrogate before the birth, Kim said: "Having a gestational carrier is definitely different, but anyone who says or thinks it's the easy way out is completely wrong". "But as much as I hated it, I still wished I could have done it on my own".
Kim said on the show: 'If the two doctors that I trust have told me it wouldn't be safe for me to get pregnant again, I have to listen to that'. He's like stuck in seven.