The 25-year-old confirmed that model girlfriend Caterina Lopez gave birth to their son Princeton J Alexander earlier today (November 28).
"Its official at 9 30am today I became a Daddy," he tweeted. "The healthy little boy has arrived at a weight of 6lb!!!!! soooo happy right now :)))) xx."
Hamblett later shared a photo of him holding his son, who was wrapped up tight in a blanket so that only his tiny feet were visible.
PRINCETON J. ALEXANDER pic.twitter.com/a2ORJAwnDA— JJ Hamblett (@JJUJWorld) November 28, 2013
He was soon congratulated by his Union J bandmates Jaymi Hensley, Josh Cuthbert and George Shelley.
"Congratulations JJ and Cat! New little one in the family now so proud of you both," Shelley wrote. "I'm not the youngest member anymore!! Haha Love You!"
SOOO HAPPY for JJ and Cat!! Its an amazing day and couldnt happen to 2 more amazing people. Chuffed to bits. The baby is gonna be beautiful.— Josh Cuthbert (@JoshUJWorld) November 28, 2013
Hamblett previously revealed to Digital Spy that he wanted an "unusual" name for his firstborn.
He revealed that he hopes to bring his baby on tour with Union J next year.