"WELCOME HOME JACK BLUES BIEBER," the Canadian singer posted with a photo of an infant's foot wrapped in a blanket, which has racked up more than eight million likes.
The celebrity couple announced in May that they were going to be parents, posting pictures of fashionista and businesswoman Hailey, 27, cradling her belly.
Justin Bieber, 30, first shot to global stardom as a teen pop sensation in 2009, since then churning out hits including the mega-selling "Baby", featuring rapper Ludacris, and "Somebody to Love."
His army of fans, known as "Beliebers," helped make him one of the best-selling artists of all time.
Fans were quick to congratulate the pair on Instagram, with one commenting, "Gotta play that song 'Baby' one more time."
Justin's mother, Pattie Mallette, also posted congratulations to the couple on X.
"I LOVE YOU FOREVER BABY JACK!!" Mallette wrote.
Actor Stephen Baldwin, Hailey's father, reshared Mallette's post, adding: "Amen, Congratulations to you and may God continue to bless our family."
Hailey Bieber has modeled in campaigns for major brands such as Ralph Lauren and Tommy Hilfiger.
She now has her own brand -- Rhode -- which sells skincare and makeup.
The model told US outlet GQ in October that she looked forward to being a mother, and found it funny how much others cared, calling motherhood "a private, intimate thing."
"It's something that's going to come when it comes. And it's just, honestly, at the end of the day, so hilarious how much people... care. Let me do what I want to do with my body and you guys can do what you want to do with your body," she was quoted as saying.
