It is the American actress' second child with husband Brian Hallisay, and they already have a 19-month-old daughter together, Autumn James.
The couple began dating in 2012 after both working on The Client List together, and married in late 2013.
Hewitt's rep confirmed to E! News that the 36-year-old gave birth on Wednesday.
The couple did not reveal the gender of their second child during pregnancy, but now they have a son and a daughter, we're pretty sure they're over the moon.
Although not documenting her pregnancy on social media, Hewitt was interviewed in the May 2015 issue of American Baby about the impending birth.
She said: "Pregnancy is magical. I'm grateful. It's not lost on me what a gift it is to be able to get pregnant.
"When you're pregnant, it's all about you—and that baby inside of you. Enjoy a nap; for some reason our generation thinks we can't do that! Really listen to your body. I hope to continue to be a little kinder to myself after pregnancy."