The 24-year-old had been tipped as a future England international when he represented Saracens, but he has made just one appearance for Northampton this season.
"Noah is a great addition to the squad, and I am delighted to welcome him to Kingston Park," Falcons director of rugby Dean Richards told the club's official website.
"He brings with him both club and international experience which is just what we require going into the playoffs."
Meanwhile, Cato added: "I am delighted to join Newcastle Falcons, and this is a good opportunity to get back to playing regular rugby.
"To be a part of a club with the ambition of returning back to the Premiership presents an exciting time ahead for me. To assist in realising that goal will be a fulfilling experience."
Cato joined Northampton from Saracens in 2011.