The Austria international, who can also play as a winger, joined Inter from Hertha Berlin at the start of the season but struggled to establish himself at San Siro, starting only three Serie A matches.
Lazaro was subsequently linked with Newcastle and Bundesliga leaders RB Leipzig, and he became the Premier League club's second January signing – after Nabil Bentaleb.
Bentaleb joined on loan from Schalke with an option to buy, but Newcastle confirmed only the temporary deal for Lazaro – despite reports he could sign for £20million (€23.5m) at the end of the campaign.
"I'm very happy to be here and can't wait to meet all my team-mates, get out on the training pitch and hopefully get a lot of victories in the future," Lazaro told the Magpies' official website.
"I've only heard great things about the club. I've had good talks with the coach, so I feel very happy to be here now. Those conversations convinced me that this is the right place for me to come."
Lazaro's opportunities at Inter were further hindered this month by the arrivals of wing-backs Ashley Young and Victor Moses.
Meanwhile, Newcastle have also been linked with a move for Tottenham left-back Danny Rose following injuries to Jetro Willems and Paul Dummett.
Head coach Steve Bruce told reporters on Friday: "He's a quality player and a class act, the kid. Whether it's possible, who knows?"