Bonorino, who has agreed a one-year contract, has won 15 caps for the Pumas and was part of the squad that reached the semi-finals of the 2007 Rugby World Cup, where he played against France and Georgia.
“I have a big friend at the Saints in Ignacio Fernandez Lobbe and he has spoken a lot of Northampton," Bonorino said.
“They have a young squad that will get better with more experience and I hope they will take another step forward next year.
"It has been a good experience at Leicester but I want to show I can do well in English rugby and play more in the Premiership."
Director of rugby Jim Mallinder said: “Although we have promising youngsters like Jon Vickers and Regardt Dreyer, we also need players who have the know-how that comes from a long career, which is what we are losing with the retirements of Barry Stewart and Tom Smith.
"Santiago is a quality player who has plenty of experience at international level and at club level across Europe."
