The This Morning presenter and her husband Dan Baldwin welcomed their third child on Monday and announced his arrival on Twitter.
More on Parentdish: Find out what Katie Hopkins thinks of Holly's baby name choice
Holly, 33, told her 4.88 million followers:
I'm sooo happy to tell you all - we've just had a baby boy! Chester William Baldwin made a grand entrance at 8lbs 12. He's just perfect. X
- Holly Willoughby (@hollywills) September 29, 2014
Celebrity friends Tweeted back their congratulations.
Amanda Holden, who's covering Holly's maternity leave on This Morning wrote:
And Holly's co-presenter Phillip Schofield added:
Congratulations and much love to @hollywills Dan Belle Harry and new baby Chester !!I'm delighted and thrilled!!
- OfficialAmandaHolden (@Amanda_Holden) September 29, 2014
Davina McCall wrote:
Had a lovely phone call from @hollywills tonight. Congratulations gorgeous and welcome Chester William Baldwin .... 8lbs 12 blimey!
- Phillip Schofield (@Schofe) September 29, 2014
@hollywills Chester!!!!!!! 😃😃😃👍👍👍👍 what a great name!!!! Well done!!!!! Hugs xxxxx
- Davina McCall (@ThisisDavina) September 29, 2014
Coleen Rooney wrote: "@hollywills congratulations!! Fantastic news!! Lots of love cxx,' while Rochelle Humes added: 'How lovely Congrats to you and Dan xx."
Holly only started her maternity leave from This Morning on September 18. It was thought her baby was due in October.
Holly and Dan, who married in 2007, announced they were expecting another child in March.
Holly wrote at the time: "Surprise Surprise (and it was a surprise!!) I'm really happy and excited to tell you that Dan and I are expecting baby number 3!! (sic)."
Last year, Holly said: "I would like another baby, so let's wait and see. I've always thought I'd have a big family so if I'm lucky enough to fall pregnant again, great.
"It's just a question of when and if it happens. You can't set aside time specially."
According to our Baby Names Generator, Chester means 'lives at the camp of soldiers' and William means 'resolute protector'. The couple also have five-year-old son Harry and three-year-old daughter Belle together.
More on Parentdish: Holly Willoughby and her sister Kelly talk to Parentdish