The couple, whose son Noah will turn two in August, announced the news in a video posted on the singer's Facebook page, reported People Magazine.
The clip, which features a soundtrack of Britney Spears' song "Oops I did it again", shows Noah point to his mother's stomach and say "baby" after Michael asks him "who's in there?"
The singer also reassured fans that Noah has recovered after being taken to hospital following a scalding water accident, last week.
The "Crazy Love" hitmaker married Lopilato in March 2011 in Buenos Aires.
"Me and my wife would really love to have three or four, five (kids), if we're blessed to do that. And if we can't do it naturally, then we'll do it another way - we'll adopt," Buble said.