The Irish singer's wife Georgina gave birth to a little girl called Gia yesterday (23 October) and they are both "thrilled" with their new arrival.
Taking to Twitter, Nicky told dans: "Our beautiful Baby Girl Gia.. Was born today at 10.30am. She is absolutely beautiful, just like her mummy! We are thrilled. Nico, G, Rocco & Jay."
Nicky and Georgina's two sons, twins Rocco and Jay, are aged six.
Former Westlife bandmate Shane Filan tweeted a message of congratulations. He said: "Huge congrats to @GginaAhernByrne & @nickybyrneoffic on the arrival of their little princess Gia.
"Amazing news ! Can't wait to see her!!"
Nicky and Georgina, who have been married for 10 years, annnounced they were expecting their third child on 29 April.
Nicky tweeted at the time: "Morning y'all... It's time to share our exciting news.. Georgina is expecting! Yep, third 'Baby Byrne' is on the way!"
See who else has given birth this year in our gallery below:
Copyright: Love & Lust London