The former Edinburgh man has been without a team since leaving the capital club at the end of last season.
He was on trial at Clermont last month but failed to land a contract with the French giants.
Newcastle moved for the back-row forward after learning that club captain Will Welch will be sidelined for two months due to an ankle injury.
Hardie, 30, has 16 Scotland caps, the most recent coming against Fiji in June of last year.
He was suspended by Scottish Rugby in November last year for three months for “gross misconduct”.
Born and raised in New Zealand, he played his National Provincial Championship Rugby with Southland and won the Super Rugby title during his time with the Highlanders. He joined Edinburgh in 2015.
Newcastle Falcons director of rugby Dean Richards said: “John is a proven performer at a very high level and it’s great to be able to call upon all of his experience and ability.
“It’s no secret that injuries and international call-ups have depleted our back-row stocks, and it’s great that we are able to welcome John to the club.”
Hardie said: “It’s fantastic to be here in Newcastle and to have this opportunity.
“It’s a fresh start for me, I’m determined get back on the rugby field and to begin this new chapter.”