No other details have been provided by the parents, who already have a two-year-old daughter, Luna May.
The TV star and model tweeted "Somebody's herrrrrrre!" along with two baby bottle emojis and two smiling face emojis.
The "Sports Illustrated" model took to Twitter to announce her and husband John Legend's second child had arrived in the world.
"I don't think Luna has a concept of exactly what it means to have a little sibling yet", Legend said.
Months later, Teigen confirmed that she was pregnant with a baby boy, which wasn't a surprise to fans of the couple. "Wanna hold my boy let's do thisssssss". It feels worse for me because with IVF you know *so* early.
She wrote: "Happy pregnant Chrissy is gone. meet OVER IT Chrissy".
The couple announced they were expecting a baby boy last November after Chrissy underwent IVF treatment.
"Baby", Luna said, uncovering her mom's baby bump.
Chrissy and John met in 2007, and dated for four years before they got engaged in 2011.
The couple married in 2013.