The former NSYNC singer already has a 5-year-old son named Silas with actress Biel, whom he married in 2012.
Timberlake broke the news in an interview, which will air in full on “The Ellen DeGeneres Show” on Monday.
In a preview on social media, DeGeneres remembered learning through FaceTime that the couple was expecting and said to Timberlake: “You are a father again, you have another small child.”
Timberlake, who turns 40 this month, told DeGeneres that she was “one of the first people outside our immediate family that we count”.
When asked if he would share the baby’s name, Timberlake said, “His name is Phineas and he is amazing. And he is so cute. And no one is sleeping. But we are thrilled. We are thrilled and we couldn’t be happier, really grateful. . “