JJ, 25, revealed on Twitter that he and his girlfriend Caterina Lopez are expecting their first baby.
"Guys I want you to be the first to know that I've got some big news," tweeted JJ to his 126,200 followers. "I'm going to be a daddy.... :-) :-) wooop x."
"My girlfriend and I are super super excited and the boys cant wait to be uncles!" Added JJ. "They are already thinking of names begining with J lol :-) x"
JJ Hamblett (pictured far left) with Union J
Mum-to-be and model Caterina shared an Instagram photo on her Twitter account of an ultrasound scan of their unborn baby.
"Someone looks extremely comfortable ...Our little monkey," she wrote.
JJ's Tweets were met with a host of excited responses from his bandmates, friends and fans.
"Big congrats to @JJUJWorld and Caterina!" Tweeted new dad Marvin Humes of JLS. "Amazing news that you're expecting your first baby..it's the BEST feeling ever! X."
"JJ THE DAD! So proud of you brother," wrote Union J's George Shelley. "It's going to be the most gorgeous baby ever! Can't wait to teach it guitar! Hang on... NAPPIES! Eww ;)"
JJ and Caterina became an item after meeting the nightclub where she worked and the couple now live together with their dog Lola.