It was rumoured in July 2020 that the couple, who are already parents to son Silas, five, had celebrated the arrival of another child.
The 39-year-old singer and actor confirmed the news during an appearance on The Ellen DeGeneres Show, in an interview which will air on Monday.
DeGeneres told him: “I think we were FaceTiming and you said ‘Hey, you want to know a secret?’ and then Jessica walked in and you put your hand on her stomach. You were like ‘We’re having another baby!'”
Timberlake replied: “You’re one of the first people outside of our immediate family that we told.”
Asked to share “vaguely his name and vaguely how old he is”, Timberlake said: “Yeah, I’ll tell you his name. His name is Phineas. He’s awesome and he’s so cute and nobody’s sleeping.
“But we’re thrilled. We’re thrilled and couldn’t be happier. Very grateful.”
The couple, who married in 2012, made headlines in December 2019 when Timberlake publicly apologised after he was pictured holding hands with a co-star.
He was photographed hand in hand with Alisha Wainwright in New Orleans while filming Palmer.
Timberlake apologised to Biel and insisted nothing had happened between him and Wainwright.
He wrote on Instagram: “A few weeks ago I displayed a strong lapse in judgement – but let me be clear – nothing happened between me and my costar.
“I drank way too much that night and I regret my behaviour. I should have known better. This is not the example I want to set for my son.
“I apologise to my amazing wife and family for putting them through such an embarrassing situation, and I am focused on being the best husband and father I can be.
“This was not that.”
Timberlake is among the performers in the star-studded line-up for the inauguration of Joe Biden on Wednesday.
Also appearing at the ceremony will be Lady Gaga, Jennifer Lopez, Jon Bon Jovi, Bruce Springsteen, Demi Lovato and John Legend.