The 40-year-old singer shared the news on Twitter alongside a selfie with their newborn son Elias Bublé.
The couple are already parents to two-year-old Noah.
"And then there were four," Bublé wrote on 22 January. "Introducing Noah's little brother Elias Bublé #family #blessed #brothers."
Lopilato then reposted the same family selfie on her Instagram account with the same caption.
Bublé announced that he and Lopilato were expecting their second child with an adorable Facebook video of Noah in July 2015.
The clip starred Noah wearing bright yellow Minion pyjamas pointing at his mum's belly, with Bublé asking: "Hey, who's that in there?"
Subtly playing in the background was Britney Spears' 'Oops I Did It Again'.
The dad-of-two then had fans guessing if they were expecting a boy or girl, before revealing in October 2015 they were expecting their second son.
The couple wed in March 2011 and Lopilato gave birth to their first child Noah in August 2013.
Congratulations to the family!