The 'Skyscraper' singer welcomed her third child at 10.41am on Wednesday, 10 September and both mum and baby are "doing well".
A name has not been announced.
Sam, 37, and husband Craig Pearson are already parents to son Tommy and daughter Brooke, who are no doubt thrilled to have a little sister!
Sam was performing on stage as recently as last month, wowing audiences at Brighton Pride Festival before going on maternity leave.
She tweeted on 3 August: "I AM OFFICIALLY ON MATERNITY LEAVE AS OF TODAY XX :-)."
The singer announced her third pregnancy in March, famously declaring that she'd discovered she was expecting after taking a pregnancy test in the toilets of her local supermarket while shopping!
"I went in and did it while he was getting all the food," she revealed. "I came out and went, 'Yep, I'm pregnant'. It wasn't planned."
Sam's pregnancy meant a planned solo tour for October this year was postponed until January 2015.
She won the tenth series of X Factor in December 2013 under the tutelage of Sharon Osbourne, who has agreed to be her baby's godmother!
On 12 March, Sharon tweeted at Sam: "I would be honoured to be your baby's Godmother!" Sam then replied: "Wow… I am so pleased. Thank you. I am one very lucky lady to have you in my life Mrs O."
Congrats to Sam and Craig!
Stars who welcomed babies in 2014:
Copyright: Rex Features WENN/Matthew Taylor/Daniel Deme/Josiah True/Matthre Taylor/Joe
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